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An H aerial is an aerial array comprising a dipole and one reflector.
 Research H Aerial
The Haber process (named after Fritz Haber) is an industrial process in which ammonia is manufactured by direct combination of its elements, nitrogen and hydrogen. The reaction is carried out at 400-500 degrees centigrade and at 200 atmospheric pressure. The two gases, in the proportions of 1:3 by volume, are passed over a catalyst of finely divided iron. Around 10% of the reactants combine, and the unused gases are recycled. The ammonia is separated by either dissolving in water or cooling to liquid form.
Research Haber process
The haboob is a violent and oppressive seasonal wind blowing in the Sudan, that brings sand from the desert.
Research Haboob
Hackman is a powerful hex editor and disassembler computer program for the IBM PC that offers many customisation options. Features include a coder, a decoder, a disassembler, watches, and five monitoring tools (including binary and octal).
Research Hackman
A hacksaw is a saw with a narrow-bladed fitted in a frame used for cutting metal. Small hacksaws are often referred to as a 'junior hacksaw'.
Research Hacksaw
In the atmosphere, a Hadley cell is a vertical circulation of air caused by convection. The typical
Hadley cells occur in the tropics, where hot air over the equator in the inter-tropical convergence zone rises, giving the heavy rain associated with tropical rainforests. In the upper atmosphere this now dry air then spreads north and south and, cooling, descends in the latitudes of the tropics, producing the North and South tropical desert belts. After that, the air is drawn back towards the equator, forming the North-East and South-East trade winds.
Research Hadley cell
In physics, a hadron is a subatomic particle that experiences the strong nuclear force. Each is made up of two or three indivisible particles called quarks. The hadrons are grouped into the baryons (protons, neutrons, and hyperons) and the mesons (particles with masses between those of electrons and protons).
Research Hadron
Haematoxylin is the colouring principle extracted from logwood. It forms yellow crystals that are slightly soluble in cold water. It is chemically of a phenolic character, and on oxidation becomes converted into a red substance, haematin.
Research Haematoxylin
Hafnium is a silvery metal element with the symbol Hf occurring in zircon and used in nuclear reactors in the control rods and also used for light-bulb filaments. It was named in 1923 by the Dutch physicist Dirk Coster and Hungarian chemist Georg von Hevesy after the city of Copenhagen, where the element was discovered (Hafnia is Latin for Copenhagen).
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Hail is a precipitation in the form of pellets of ice, known as hailstones. It is caused by the circulation of moisture in strong convection currents, usually within cumulonimbus clouds. Water droplets freeze as they are carried upwards. As the circulation continues, layers of ice are deposited around the droplets until they become too heavy to be supported by the currents and they fall as a hailstorm.
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Hair Compasses are compasses having a spring tending to keep the legs apart, and a finely-threaded screw by which the spring can be compressed or relaxed with the utmost nicety, and the distance of the legs regulated to a hair's-breadth.
Research Hair Compasses
In mechanical watchmaking, a hair-spring is the fine hair-like spring made of steel, which is attached to the axle of the balance wheel, and serves by its resisting power to equalize the vibrations of the escapement-wheel.
Research Hair-Spring
In chemistry, half-life is the length of time required for one-half of a radioactive substance to disintegrate.
Research Half-life
In chemistry, a halide is a compound composed of two elements, one of which is a halogen.
Research Halide
The Hall effect is when a piece of semiconductor material with a current flowing through it is subjected to a magnetic field a voltage is set up between the faces of the material which are perpendicular to both the current and the field. It is caused by the charge carriers present in the semiconductor being deflected in the magnetic field.
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Halo is the name given to coloured circles of light sometimes seen round the sun or moon, and to other connected luminous appearances. These phenomena are classified as: (1) Halos proper, consisting of complicated arrangements of arcs and circles of light surrounding the sun or moon, accompanied by others tangent to or intersecting them; (2) coronas, simple rings, generally somewhat coloured; (3) aureolas, the name given to the kind of halo surrounding a shadow projected upon a cloud or fog-bank, or to the coloured rings observed by aeronauts on the upper surface of clouds. All these appearances are the result of certain modifications which light undergoes by reflection, refraction, dispersion, diffraction, and interference when it falls upon the crystals of ice, the raindrops, or the minute particles that constitute clouds.
Research Halo
Halogen is a particular group of elements with similar bonding properties.
Research Halogen
In chemistry, the halogens are a family of elements consisting of fluorine, chlorine, bromine and iodine.
Research Halogens
Halons are organic chemical compounds containing one or two carbon atoms, together with bromine and other halogens. The most commonly used are halon 1211 (bromochlorodifluoromethane) and halon 1301 (bromotrifluoromethane). The halons are gases and were widely used in fire extinguishers until they' re use was banned in 1994 because of the damage they cause the ozone layer.
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HAM-RPM is a knowledge-based conversationalist that reasons with fuzzy information. It was developed at the University of Hamburg.
Research HAM-RPM
In computing, handshaking refers to hardware or software activity designed to start or keep two machines or programs in synchronisation as they do protocol.
Research Handshaking
Handy Quote is a sales-quotation, point-of-sale (POS), inventory control, sales ordering, purchase ordering and sales management computer program designed for any type of businesses. Handy Quote is a fully integrated software package that provides a complete solution to your fast moving business requirements. It contains help files, and an extensive list of features provided.
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In steam engines, a hanging-bridge was a hollow, vertical partition depending from the bottom of a boiler and serving to deflect the flame.
Research Hanging-Bridge
Harrison's timepiece was initially produced by John Harrison of Foulby in 1735 in response to a British government offer of reward in 1714 for methods of determining longitude at sea; to obviate the irregularities in the rate of timepieces by variations of temperature. A second and third timepiece followed and finally a fourth which won for Harrison the prize offered by the British government of 20,000 pounds sterling.
Research Harrison's Timepiece
Harrows are various distinct agricultural instruments having teeth. A break harrow, usually with teeth at an angle of 80 degrees, was used for finely pulverising the soil preparing it for sowing; angular weeding harrows sometimes followed the break harrow and were designed to uproot and collect weeds, they had rearmost teeth at a more acute angle to collect the uprooted weeds; a third type of harrow consisting of a one or two frames of teeth attached by cord to a wheeled harness - and thereby applying a gentle raking action to the soil - was used for covering the sown seeds in the drill.
Research Harrow
A Hartley oscillator is a thermionic oscillator comprising a triode with an oscillatory circuit connected between anode and grid, and inductive coupling between the anode and grid circuits, via a tapping on the inductive element of the tuned circuit.
Research Hartley Oscillator
Harvard Graphics by Software Publishing Corporation, was an easy-to-use, integrated business- presentation program for personal computers that produced text charts, organisation charts, and graphs including pie, bar/ line, clustered, area, high-low-close, three-dimensional, stacked bar three dimensional, and scatter charts. Charts were generated from imported data or data keyed into the program.
Harvard Graphics provided many tools to save time. Speed keys were used to speed up common functions such as printing and saving a chart. Templates allowed the user to select chart attributes once and use them repeatedly. Chartbooks held related templates. An automatic data link linked a Lotus 1-2-3 or compatible spreadsheet to a chart template. Data in 1-2-3 or Excel spreadsheets could be selected using range names for speed and convenience. Macros automated data entry or any repetitive task. A DOS Shell allowed access to up to seven other applications without exiting from Harvard Graphics.
Harvard Graphics let you incorporate text into charts and annotate them with freeform symbol drawings. A link to Harvard Graphics Draw Partner gave direct access to drawing tools for modifying charts. You could edit individual elements of your drawing as well as pieces of a chart. The program included hundreds of pre-drawn symbols and icons to add to your graphics. A built-in spell checker was very useful before finalising a presentation.
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In weaving, a heck-box is a box suspended between the traverse on which the bobbins of warp yarn are mounted, and the warping frame on which the yarns are wound. It divides the warp threads into two sets, one for each heddle.
Research Heck-Box
Heckles or hackles are an apparatus employed in the preparation of animal and vegetable fibres for spinning. It consists of a series of long metallic teeth, through which the material is drawn so as to comb the fibres out straight and fit them for the subsequent operations. The teeth are fixed in a wooden, metallic or synthetic base, in several rows, alternating with each other at short distances apart.
Research Heckles
In weaving, a heddle is one of the sets of parallel knotted cords forming loops for the warp threads; and by whose vertical reciprocation the warp threads are shifted so as to make the shed for the passage of the shuttle. Heddles are a necessary integral feature of all looms, having sets of strings for separating the warp threads into two or three groups, between which the weft is passed. This called mounting the loom and consists in dividing the warp among the leaves of healds or heddles.
Research Heddle
In astronomy, heliacal is the rising or setting at the same time, or nearly the same time, as the sun. The heliacal rising of a star is when, after being in conjunction with the sun and invisible, it emerges from the light so as to be visible in the morning before sunrising. On the contrary, the heliacal setting of a star is when the sun approaches so near as to render it invisible by its superior splendour.
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A helical aerial is an aerial array composed of a conductor wound in the form of a helix, the circumference of each turn being equal to a whole number of wavelengths.
Research Helical Aerial
The heliometer was an instrument for measuring small distances on the sky, particularly the apparent diameters of the sun and of the moon. It was invented by Bouguer in 1747, and improved by Dollond and Fraunhofer. In the common later form the object-glass of the telescope was cut into two halves, relatively movable by a screw. Each half formed a perfect image in the focus of the eye-piece, and by varying the distance between the half-lenses the images could be made to diverge from, or approach, each other. If, in contemplating a celestial body, the object-glasses were placed so as to bring the images to touch each other, the distance of the centres of the object-glasses, measured in seconds, gave the diameter of the image.
Research Heliometer
A helioscope is a telescope fitted for viewing the sun without damage to the eyes, as when the image of the sun is received upon mirrors formed simply of surfaces of transparent glass which reflect only a small portion of the light.
Research Helioscope
The heliostat is an instrument invented by Gauss in 1821, by means of which rays of the sun can be flashed to great distances. It consists of an adjustable mirror or reflector, worked in connection with a combination of telescopes and was formerly used in trigonometrical surveys allowing measurements of hundreds of miles to be made.
Research Heliostat
Heliotype is a photographic process by which pictures can be printed in the same manner as lithographs, depending on the fact that a dried film of gelatine and bichromate of potash, when exposed to light, is afterwards insoluble in water, while the portion not so exposed swells when steeped. A mixture of gelatine, bichromate of potash, chrome alum, and water was poured on a plate of glass, where it shortly settled into a film. When dried the film contracted and separated from the glass. A picture was then printed on it from a negative, after which it was attached to a plate of zinc, and copies were taken from it by inking it with lithographic ink exactly as in the ordinary lithographic process. The films were technically called 'skins'. Sometimes a gutta-percha mould was prepared from the film, and copper deposited on it by the electrotype process, the plate thus produced being printed from in the ordinary way.
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Helium is a gaseous element of the noble gas group with the symbol He.
Research Helium
Henry is the unit of inductance. An inductor has an inductance of one henry if an electro-motive force of one volt is induced when the current changes at the rate of one ampere per second.
Research Henry
Heptachlor is a man-made compound that was commonly used by American exterminators and home owners to control and kill termites, and by farmers to kill insects in seed grains and on crops.
Heptachlor epoxide is an oxidation product of heptachlor formed by many plants and animals, including people, after exposure to heptachlor. Heptachlor is present as an impurity in the pesticide chlordane. However, since late 1978, most uses of heptachlor have been phased out; the chemical is no longer available to the American general public, and as of April 1988, heptachlor can no longer be used for the underground control of termites. Heptachlor is a crystalline solid when it is pure, and a waxy solid as a technical-grade product. Heptachlor epoxide is a solid.
Heptachlor is soluble in water; heptachlor epoxide is insoluble. As a pure compound, heptachlor is a light tan solid that smells some-thing like camphor.
Heptachlor is also known as: 1,4,5,6,7,8,8-heptachloro-3a,4,7,7a-tetrahydro-4,7-methanoindene;
heptachlorodi-cyclopentadiene. Heptachlor epoxide is also known as 1,4,5,6,7,8,8-heptachloro-2,3-epoxy-2,3,3a,7,7a-hexahydro-4,7-methanoindene; epoxyheptachlor.
Research Heptachlor
A heptagon is a seven-sided polygon.
Research Heptagon
A heptode is a high-vacuum thermionic valve having seven electrodes, namely an anode, a cathode and five grids.
Research Heptode
The Hercules Graphics Card or HGC was a revolutionary video adapter for Personal Computers introduced in 1982 by the Hercules company. Like the existing MDA and CGA graphics adapters the Hercules Graphics Card was based upon the Motorola MC6485 video controller but could display two 720 x 348 resolution graphic screen pages in monochrome.
Research Hercules
Hermeneutics (from a Greek word meaning to explain or interpret), is the science which fixes the principles of the interpretation of the sacred writing's. Hermeneutics bears the same relation to exegesis as theory to practice.
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Herpetology (from the Greek herpeton, a reptile) is that department of natural history which deals with reptiles.
Research Herpetology
In chemistry, heterocyclic compounds are cyclic compounds in which the ring system of the molecule contains other elements than carbon.
Research Heterocyclic compounds
Heterodyne is the process whereby two oscillations of different frequencies are combined to produce other oscillations, and particularly oscillations having a frequency equal to the difference between the frequencies of the two original oscillations.
Research Heterodyne
Heuristic Dendral is an expert system, developed at Stanford University, that establishes the structure of a molecule given the molecule's atomic formula and mass spectrogram.
Research Heuristic Dendral
A hexagon is a six-sided polygon.
Research Hexagon
A hexahedron is a figure having six faces, or a solid bounded by six planes. The term cube is now generally applied to the regular hexahedron.
Research Hexahedron
Hick's mandril is an arbor for turning rings; at the centre of the arbor there is a cone, round which, at equal distances, wedges are fitted into dove-tailed grooves, and are expanded to the bore of the ring by a nut acting on a screw at the end of the cone.
Research Hick's Mandril
In computer terms, hidden refresh allows PC RAM refresh memory cycles to take place in memory banks not used by the CPU at the time, instead or together with the normal refresh cycles, which are executed every time a certain interrupt (DRQ0 every 15 msecs) is called by a certain timer (OUT1). Every time it takes two to four msecs for the refresh. There are typically three types of refresh schemes: cycle steal, cycle stretch, or hidden refresh. Cycle steal actually steals a clock cycle from the CPU to do the refresh. Cycle stretch actually delays a cycle from the processor to do the refresh. Since it only occurs every say 4ms or so, it's an improvement from cycle steal. We're not really stealing a cycle, only stretching one. Hidden refresh typically doesn't stretch or steal anything. It's usually tied to DTACK (Data acknowledge) or ALE (Address Latch Enable) or some other signal relating to memory access. Since memory is accessed all of the time, ie RAM, ROM, etc. it's easy to synchronize the refresh on the falling edge of this event. Of
course, the system performance is at its optimum efficiency, refresh wise since the RAM is not taking clock cycles away from the CPU.
Research Hidden Refresh
High Tension is a comparative term used in electronics to denote high voltages.
Research High Tension
Hijaak is a graphics accessory program that lets you convert graphic files from one format to another. The program captures screen images and graphics data (both text and data) in CGA, EGA, VGA, ATT, DEB, and Hercules graphics resolutions, and converts graphic files from one format to another on a variety of machines including the IBM PC, Macintosh, and Amiga. HiJaak is ideal for desktop publishing or presentation graphics users who want to move a graphic image into a document where the formats are incompatible. For example, bringing a vector-based drawing or CAD picture into a painting package or bringing a Mac paint image into an IBM paint package.
Research HiJaak
Histology is the study of the tissues which enter into the formation of animals and plants, and their various organs, by means of the microscope and chemical and physical reagents. It may be described as a kind of minute anatomy. It comprehends the structure and mode of development of the various tissues, and is divided into animal histology and vegetable histology.
Research Histology
A hodometer was an early device for measuring the distance travelled by a vehicle. It consisted of a clockwork arrangement fixed to the side of the vehicle, and connected with the axle. An index recorded on a dial the distance travelled.
Research Hodometer
A hoe is an instrument for cutting up weeds and loosening the earth in fields and gardens. In shape the hoe is something like an adze, being a plate of iron or steel, with an eye for a handle, which is set at a convenient angle with the plate. The Dutch hoe differs from the common hand hoe in having the cutting blade set like the blade of a spade. A horse-hoe is a frame wheel-mounted, and furnished with ranges of shares spaced so as to work in the intervals between the rows of turnips, potatoes, etc. It is used on farms for the same purpose as the hand hoe, and was formerly pulled by horse-power.
Research Hoe
The holophote was a form of lamp in which the light was converged and directed to a particular spot to prevent collisions at sea etc.
Research Holophote
In geometry, homologous means corresponding in relative position and proportion.
Research Homologous
Hooke's law (formulated by Robert Hooke) states that: Provided the elastic limit is not exceeded, the deformation of a material is proportional to the force applied to it.
Research Hooke's Law
In brewing, a hop-press is a machine for expressing the liquid from hops after boiling.
Research Hop-Press
A hopper is a chute for feeding any material to a machine. They are generally of an inverted conical shape, and the name derives from their use in the flour industry where the grain needed to be shaken down the hopper to keep it flowing.
Research Hopper
Horizon-glasses are the two speculums on of the radii of a quadrant or sextant. The one half of the fore horizon-glass is silvered, while the other half is transparent, in order that an object may be seen directly through it. The back horizon-glass is silvered above and below, but has a transparent stripe across the middle, through which the horizon can be seen.
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Horology is the science dealing with the study of time and timepieces.
Research Horology
Horsepower is an imperial unit of power, now replaced by the watt. It was first used by the engineer James Watt, who employed it to compare the power of steam engines with that of horses - one horsepower being the force with which a horse acts when drawing. The mode of ascertaining a horse's power was to find what weight he could raise and to what height in a given time, the horse being supposed to pull horizontally. From a variety of experiments of this sort it was found that a horse, at an average, can raise 160 lbs weight at the velocity of 2.5 miles per hour. The power of a horse exerted in this way was made the standard for estimating the power of a steam-engine. Thus people formerly spoke of an engine of 60 or 80 horsepower, each horsepower being estimated as equivalent to 33,000 lbs. raised one foot high per minute.
Engineers differed widely in their estimate of the work a horse is able to execute. The figure given is the estimate of Boulton and Watt and was based on the work of London dray-horses, but it was considered much too high, 17,400 foot-pounds per minute being generally considered nearer the truth. As it matters little, however, what standard be assumed, provided it be uniformly used, that of Watt was generally adopted. The general rule for estimating the power of a steam-engine in terms of this unit was to multiply together the pressure in pounds on a square inch of the piston, the area of the piston in inches, the length of the stroke in feet, and the number of strokes per minute, the result divided by 33,000 gave the horse-power, deducting one-tenth for friction. As a horse can exert its full force only for about six hours a day, one horse-power of machinery is equal to that of 4.4 horses.
In the UK, one horsepower is now equal to 550 foot-pounds per second or 745.7 watts. In the USA this figure has been rounded to 746 watts, and in the metric system it is 735.5 watts.
Research Horsepower
In a lathe, the horseshoe is a movable support for varying the gearing and the velocity of the screw which moves the slide.
Research Horseshoe
Host is a computer program for the Unix platform that looks for information about Internet hosts.
Hosts can be specified as either a numerical address (e.g.: 125.10.65.19) or a name (e.g.: probertencyclopaedia.com).
Research Host
In computing, hot linking refers to web pages which cause an object, usually an image, to be displayed which is located on another web server. This often causes problems for the owner of the other web server as they have to pay for the transfer of the object from their server to the client, for the benefit of the hot linking web site. With the popular Apache web server, hot linking of objects is simply prevented by configuring the Apache server, most usually be adding a '.htaccess' file with suitable commands to the directory in which objects that should not be hot linked are located.
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Hot-metal typesetting is a method of setting type in which the type itself is cast from molten metal. Systems include the Monotype and Linotype machines. Hot-metal typesetting has largely been replaced by phototypesetting, which can more easily be computer-controlled.
Research Hot-Metal Typesetting
An hourglass is an instrument consisting of two glass bulbs placed one above the other joined by a narrow passage through which a substance, usually grains of sand, trickle at a uniform rate, taking an hour for all the contents to pass from the top bulb through to the lower bulb. The hourglass was commonly used in churches during the sixteenth and seventeenth centuries to regulate the length of the sermon. Smaller models are frequently sold as egg timers, taking a few minutes for the substance to pass from one bulb to the other.
Research Hourglass
In telecommunications, HSP (Host Signal Processor) is a modem that depends on the host CPU (the Pentium, PowerPC, etc., in the main computer) for part or most of the data processing.
Research HSP
HTML (Hypertext Markup Language) is an authoring language which was invented at the European Particle Physics Laboratory (CERN) during the late 1960s as a distribution system for creating and sharing multimedia-enabled, integrated electronic documents over the Internet. HTML was developed to unify text, pictures and sound into a single document with hypertext linking whereby documents automatically referenced other documents, saving readers from manually referencing related documents. The first truly successful viewer for HTML documents was developed at the National Center for Supercomputing Applications (NCSA) at the University of Illinois, it was called Mosaic and was distributed free. With the birth of Mosaic came the World Wide Web. The developers of Mosaic at NCSA left and established the company Netscape, producing the popular Netscape Navigator browser, which like almost all modern web browsers is based on the original Mosaic.
Research HTML
HTTP is a protocol used to transmit HTML format documents between servers and web browsers.
Research HTTP
HTTrack is a free computer program for the Windows and the Linux operating systems by Xavier Roche, designed to be an easy-to-use offline web browser utility. It enables one to download a website from the Internet to a local directory, recursively building the original website directory structure, retrieving all html, image, and other files from the server and copying them to a local computer. HTTrack arranges the original site's relative link-structure, so that a functioning local copy of the original website is produced. Httrack enables the user to select which user-agent will be declared to the target web server, thereby preventing a webmaster from blocking Httrack.
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 More information about HTtrack
Humus is a substance which occurs in vegetable mould, and in liquids containing decomposing vegetable matter. Humus as it exists in the soil is a product of the decay of vegetables. It is a mixture of various carbon compounds, which slowly undergo combustion with the production of carbon dioxide, water, and ammonia, which are again taken up by plants.
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In gearing, a hunting-cog is a tooth in a cog-wheel which is one more than a number divisible by the number in the pinion with which it engages. If the pinion contains eight leaves and the wheel 65 teeth, the 65th , or hunting-cog, prevents the recurrence of each leaf of the pinion with every eighth cog of the wheel, which would be the effect were the relative numbers eight and 64. When the numbers are eight and 65, the wheel will revolve eight times and the pinion 65 times before the same leaves and teeth will again be engaged.
Research Hunting-Cog
In mining, hushing is a mode of exposing and collecting ore whereby water is allowed to flow through a ravine so as to wash away surface soil and expose ore below.
Research Hushing
Hyacinth is a shade of purple in which blue predominates.
Research Hyacinth
In chemistry, a hydrate is a compound that has discrete water molecules combined with it. The water is known as water of crystallization and the number of water molecules associated with one molecule of the compound is denoted in both its name and chemical formula.
Research Hydrate
In chemistry, hydration is the combination of water and another substance to produce a single product. It is the opposite of dehydration.
Research Hydration
In earth science, hydraulic action is the erosive force exerted by water (as distinct from the forces exerted by rocky particles carried by water). It can wear away the banks of a river, particularly at the outer curve of a meander, where the current flows most strongly.
Hydraulic action occurs as a river tumbles over a waterfall to crash onto the rocks below. It will lead to the formation of a plunge pool below the waterfall. The hydraulic action of ocean waves and turbulent currents forces air into rock cracks, and therefore bring about erosion by cavitation .
Research Hydraulic Action
The hydraulic press is a type of machine based upon Pascal's principle, that is when a fluid completely fills a vessel, and a pressure is applied to it at any part of its surface, that pressure is transmitted equally throughout the whole of the enclosed fluid. The
hydraulic press has numerous uses, from the compression of soft materials such as waste paper and cotton into compact bales to the shaping of motor-car bodies and the forging of steel armour plate. In its simplest form the hydraulic press consists of a cylinder and piston of large diameter, connected by a pipe to a force pump of much smaller diameter. Oil from a supply tank is pumped into the cylinder and the piston (or ram) moves out, exerting considerable force. A valve is provided to release the pressure and allow the oil to return to the tank, after the press has done its work.
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The hydraulic radius is a measure of a river's channel efficiency, and is used by water engineers to assess the likelihood of flooding. The hydraulic radius of a channel is defined as the ratio of its cross-sectional area to its wetted perimeter. The greater the hydraulic radius, the greater the efficiency of the channel and the less likely the river is to flood. The highest values occur when channels are deep, narrow, and semi-circular in shape.
Research Hydraulic Radius
Hydric is a chemistry term given to acids which are regarded as salts of hydrogen.
Research Hydric
In chemistry, a hydride is a compound containing a negatively charged hydrogen, as in sodium
hydride(Na-H).
Research Hydride
A hydrocarbon is a chemical containing only hydrogen and carbon.
Research Hydrocarbon
Hydrochloric acid (also known as chlorhydric acid, muriatic acid and spirit of salt) is a compound of chlorine and hydrogen that occurs in the human stomach. It was discovered in 1648 by Johann Glauber and first prepared in 1772 by Joseph Priestley, while its constitution as a compound solely of chlorine and hydrogen was determined in 1810 by Humphry Davy. Today the term is restricted to the aqueous solution of hydrogen chloride (which is a gas normally).
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Hydrocyanic Acid (Prussic Acid) was discovered by Scheele in 1782, and first prepared in a pure state by Gay-Lussac in 1811. It is found in the kernels of bitter almonds, peaches, apricots, plums, cherries and quinces and various plants leaves including beech, cherry and laurel. It is one of the most toxic substances known and is used to prepare cyanide. Hydrocyanic acid was originally known as Prussic acid on account of being the acid of Prussian Blue.
Research Hydrocyanic Acid
Hydrodynamics is the science which deals with the nature of liquids. It especially investigates the motion of liquids through orifices in tubes, or that of water in canals, rivers &c. One of the first studies into the motion of water in rivers and canals was carried out by Guglielmini in 1691.
Research Hydrodynamics
Hydroelectric power (HEP) is electricity generated by moving water. In a typical HEP scheme, water stored in a reservoir, often created by damming a river, is piped into water turbines, coupled to electricity generators. In pumped storage plants, water flowing through the turbines is recycled. A tidal power station exploits the rise and fall of the tides. About one-fifth of the world's electricity comes from HEP. HEP plants have prodigious generating capacities, for example the Grand Coulee plant in Washington State, USA, has a power output of some 10,000 megawatts. The Itaipu power station on the Parana River in Brazil/Paraguay has a potential capacity of 12,000 megawatts.
Research Hydroelectric Power
Hydrofluoric acid or hydrogen flouride is a strong acid, which, though monobasic, forms double salts of the alkali metals by union of a molecule of salt and of acid. Hydrofluoric acid is a solvent of silica and silicates, and is used for etching glass. Hydrofluoric acid is obtained in aqueous solution by heating calcium fluoride with concentrated sulphuric acid and condensing the gas given off in water.
Research Hydrofluoric Acid
Hydrogen is a non-metallic gaseous element with the symbol H. In its free gaseous state it is only found in nature in small quantities issuing from crevices in volcanic districts or near petroleum wells. It exists in combination everywhere; as a constituent of water, of all plants and animals, and in numerous minerals, abundantly in coal, petroleum, bitumen, etc., and to a lesser degree in rocks. The element may be separated from any of its compounds, but it is usually obtained from water or dilute acids.
Research Hydrogen
Hydrogen peroxide is a colourless, viscous, unstable liquid compound of hydrogen and water with the formula H2O2 with oxidising and reducing properties. Hydrogen peroxide was discovered in 1818 and was used for bleaching wool, silk, linens, furs and hair products since the 19th century. During the Second World War hydrogen peroxide was used as a propellant in V1 rockets and was subsequently also used as a propellant for submarines. Hydrogen peroxide is also used as a preservative and disinfectant and is used by archaeologists to dissolve clay from finds.
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Hydrogen sulphide is a colourless, flammable and poisonous gas with a smell like that of rotten eggs, produced naturally by putrefying organic material.
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In chemistry, hydrogenation is a chemical reaction in which hydrogen is added to a compound.
Research Hydrogenation
In chemistry, hydrolysis is chemical decomposition by which a compound is resolved into other compounds by taking up the elements of hydrogen.
Research Hydrolysis
Hydroponics is the process of growing plants on a sterile inert material such as sand, gravel, or liquid, without soil but continuously provided with nutrients in solution. In this way it is possible to control the balance of inorganic nutrients such as potassium, sulphur, magnesium, and nitrogen, supplied to the plant, and to provide the correct mixture for a particular stage of plant growth. Crops are planted in one metre wide beds fitted with movable shades to control exposure to sunlight. Hydroponics has a particular application for the production of crops such as green peppers and aubergines in greenhouses, but has also been applied on a much larger scale, particularly in desert regions such as Kuwait, where the water supply is sparse and the soil is very poor quality.
Research Hydroponics
Hydroquinone is a reducing agent used in photographic developers.
Research Hydroquinone
A hydroxide is an inorganic compound containing one or more hydroxyl groups.
Research Hydroxide
A hydroxyl group is a molecule where an oxygen atom is covalently bonded to a hydrogen atom.
Hydroxyl groups are characteristic of alcohols, phenols, and carboxylic acids. Oxygen has a higher electronegativity than hydrogen, so a hydroxyl group has a dipole moment, the hydrogen being the positive end of the dipole. This polarity determines much of the chemistry of the hydroxyl group. For example, under certain circumstances, the molecule acts as an acid, hydrogen dissociating as an H+ ion. Because of the high concentration of negative charge on the oxygen, it can act as a nucleophile, as in the formation of esters.
Research Hydroxyl Group
Hydroxylamine is a powerful reducing agent and has basic properties, forming salts by union with acids. It is a white solid that explodes when heated and dissolves in water.
Research Hydroxylamine
A hygrometer is an instrument used to measure atmospheric humidity. A simple form of hygrometer, used in houses and offices, utilises the change in length in an organic fibre (often a human hair) brought about by the absorption of moisture. The fibre tends to lengthen in damp air, and the apparatus is so arranged that the change in length of the fibre moves a pointer across a dial, which is calibrated to give a reading in percent relative humidity. This type of hygrometer gives only an approximate indication of humidity and is not used for accurate, quantitative determinations.
The instrument most commonly used in laboratories to measure relative humidity is the psychrometer (originally called Mason's Hygrometer), or the wet-and-dry-bulb thermometer. Two similar thermometers are mounted side by side; one, the dry bulb, has its bulb exposed to the atmosphere, and the bulb of the other, the wet bulb, is wrapped in a suitable material, such as muslin, which is immersed in water and serves as a wick. The wet bulb is cooled by evaporation of the water, the amount of evaporation and consequent cooling of the thermometer depending on the humidity of the atmosphere - the drier the atmosphere, the faster the water evaporates. A table accompanying the instrument gives the relative humidity in terms of the readings of the wet-bulb and dry-bulb thermometers.
Daniel's Hygrometer was a dew-point hygrometer and consisted of a bent tube with a globe at each end, and was partly filled with ether, the rest of the sapce in the tube being filled with eather vapour, all of the air having been expelled. One globe was made of blackened glass, and contained a thermometer, while the other globe was covered with muslin. Before use, the ether was made to pass into the blackened globe containing the thermometer, while the muslin surrounding the second globe was moistened with ether. This ether rapidly evaporated, causing a condensation of some of the ether vapour inside the tube; this in turn produced an evaporation of the ether in the blackened globe. Whenever evaporation occurs there is an absorption of heat, so that the black bulb gradually became colder and coleder, and the moment was reached when the air in contact with it began to deposit dew on its surface, at this point the temperature was read which indicated the dew point.
Research Hygrometer
Hygroscopicity is the taking up and giving out of moisture to and from the atmosphere.
Research Hygroscopicity
In chemistry, a hypertonic solution is a solution of higher osmotic pressure than another with which it is compared.
Research Hypertonic solution
Hypsometry or the measurement of heights, is that department of geodesy which treats of the measurement of the absolute or relative heights of various points on the earth's surface.
In all cases in which great accuracy is essential, trigonometrical methods must be employed, but in other cases sufficiently accurate results were traditionally obtained by levelling, by the use of the barometer, or by the boiling-point of water as given by the thermometer. The trigonometrical method is often the only one available, as the height to be measured may be quite inaccessible. The barometric method is based on the fact that as the mercurial column is supported by the atmospheric pressure, it must fall when conveyed from a lower to a higher level, as in the latter case the pressure is diminished.
Were the atmosphere uniform in density throughout, nothing could be simpler than the measurement of heights by the barometer, but gases being very compressible, the lower strata of the atmosphere are denser than the upper strata, being exposed to greater pressure. Thus a column of air 100 feet high, has far greater weight at the sea-level than a similar column at the top of a mountain 4000 feet high; and the effect on the barometric column of rising 100 feet from sea-level is correspondingly greater than the effect of rising 100 feet from a height of 4000 feet above the level of the. sea. Moreover, increase of temperature affects the density of the mercury in the barometer, and also that of the air, and further complicates the problem. Hence for the greatest accuracy in determining the difference of levels two mercurial barometers and four thermometers were required. Two of the thermometers were used for determining the temperature of the air at the stations, and two were attached to the barometers for determining the temperature of the mercury. The observations were made simultaneously. The aneroid barometer was in some respects more suitable than the mercurial, being much more portable, and requiring two thermometers only. After the necessary observations were made the required height could be calculated by the use of certain logarithmic formulae, or by the rough methods described in the article on barometer
Tables obviating the use of logarithms were often supplied by instrument makers along with aneroid barometers. The method in which use was made of the principle that water boils at the temperature of 100 degrees Celsius under the full pressure of the atmosphere but at a lower temperature with a smaller atmospheric pressure, such as is given by an elevated position, is simple and sufficiently accurate for many purposes.
Research Hypsometry
Hysteresis is a lag in a variable of a system, with respect to the effect causing the variation.
Research Hysteresis
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