Archive | G RSS feed for this section

GLYCURONIC ACID

A product obtained from saccharic acid by reduction with sodium amalgam.

Comments are closed

GADOLINITE

A complicate silicate mineral found in Norway and Sweden of black, brown, and yellow colour, containing the rare earths yttria, ceria, etc.; also found in pegmatites in Arizona, Colorado, and Texas.

Comments are closed

GALLIC ACID

occurs in nut galls, sumach, tea, and a number of tannins in the form of a glucoside, from which it can be made by boiling with acids. In the pure state it crystallizes in nearly colourless silky needles of the composition CjH60

Comments are closed

GLASSBLOWING

An art that can only be acquired by practice. Suppose it is desired to make a sealed tube with a bulb at one end, a piece of glass tube of the deiiired size and lengthsay t inch diameter and 4 or 5 inches in lengthis taken, and one end placcd in a Bunsen or blowpipe flame until the glass softens sufficiently to mass together, close up, and become solid to the extent of about i inch. Whilst still redhot and soft, the mouth should be applied to the open end, turning the tube round in the fingers, and meantime blowing with enough pressure to swell out the molten glass. It may be necessary, and often is so, to reheat the blownout part and repeat the blowing until a bulb of the right size and shape is obtained. Bulbtubes thus prepared are useful for observing the behaviour of solid chemical substances placed in them for that purpose when heat is applied. The conveyance to the bulbtube of the substance to be examined can be easily effected by the use of a sharply channelled slip of paper. Iodine will be seen to volatilize, and give off fumes of its own colour, and to recondense to the sold state in the upper part of the tube. Sulphur can be seen to melt and pass through the stages described under that heading, including sublimation and recondensation. Mercury can be sublimed and seen to condense on the upper cool part of the tube so also ammonium chloride. Lead filings can be melted in the bulb. White lead is decomposed, carbon dioxide being given off as gas, and yellow litharge being left behind in the tube. Glass Tpieces can be made with a little practice, and are often wanted in the laboratory. Take a piece of glass tube of the desired length, and plug one end with a small boring of cork then hold it in the flame of a blowpipe so that a fine tongue of flame impinges upon and heats the tube in one spot only, near the middle; and when it is observed to be redhot, remove the tube from the flame and place the open end quickly in the mouth. Upon blowing, the molten part will become distended into the shape of a swelling or balloon, so thin that it can easily be broken, thus lea> ing a hole in the tube, the edges of which can be rounded off with a file. Next, take another piece of glass tube and blow a bulb at one end as previously described, taking care, however, in this case to blow the bulb as large, and therefore as thin, as possible. This bulb is then to be broken and the edges rounded off as in the other case with a file, when it remains to join the two pieces together. We have then the one tube with a hole in its centre, and the other tube with one end provided with a sort of lip roughly fitting the hole as to size. The flame of the blow pipe should now be applied to both these parts, and when sufficiently softened by the heat they can be joined together in the flame.

Comments are closed

GLAZES

Fusible mixtures of felspar and other substances used in the ceramic industries.

Comments are closed

GAMBOGE

A gum resin from the Stalagmites cambogioides tree which grows in Cambogia. It acts as a drasticpurgative, but as prepared in cakes for the market it is chiefly used as a pigment for watercolour painting.

Comments are closed

GALL NUTS

are produced as excrescences on the leaves of the oak by punctures of gallwasps, in which they ay their eggs. The Aleppo galls are most valued, but although those from the Levant are the best, there are others of inferior quality which come from Dalmatia and other places. They contain tannic or gallotannic acid, which is also present in sumach and tea, and are used in tanning, in the manufacture of blueblack ink, also for the production of certain tints in Turkeyred dye ing and in calicoprinting.

Comments are closed

GASES

In addition to the recently discovered tare elements contained in the air, five of the better known ones exist ordinarily in the gaseous stateviz., hydrogen, oxygen, nitrogen, chlorine, and fluorine. Gases vary greatly in their general properties, being of varying colours, densities, solubilities in different fluids, and chemical affinities. They can all be reduced to the liquid or solid state by lowering the temperature and increasing the pressure sufficiently, and the highest temperature at which a gas can be liquefied by pressure is called its “critical temperature,” while the “critical pressure” is that under which it can be liquefied at its critical temperature. Many gases are readily absorbed by charcoal, and some of them are occluded by metals. Recently heated beechwood charcoal will absorb by what is called “surface action” go times its own volume of ammonia, while that from cocoanut shell will take up 171 volumes. The gases most easily liquefied are those which are most readily absorbed by charcoal, and in this condensed form they exhibit unusually active chemic al properties. For example, powdered charcoal saturated with hydrogen sulphide when brought into contact with oxygen, bursts into combustion owing to the rapid chemical action of the two gases. All gases tend to expand and the pressure or elastic force of a gas is the collective effect of the bombardment of its freely moving molecules against the containing vessel. A list of the critical temperatures and pressures and boilingpoints of the better known gases is appended: Equal volumes of gases at the same temperature and pressure contain an equal number of molecules, and the volume occupied by a given weight of any gas is inversely as the pressure. Thu densities of the gaseous elements are, for the most part, identical with their atomic weights. The densities of compound gases are onehalf of their molecular weights. One litre of hydrogen at o degrees C. and 760 mm. mercury weighs o08936 grm., and the weights of litres of other gaseous elements are ascertained by multiplying this factor by their atomic weights. Gases expand part of their volume at o degrees C. for every increase of i degrees C. in temperature at constant pressure. Gases exhibit a peculiar property of diffusion, so that if two vessels containing, say, oxygen and hydrogen respectively, be placed with their openings in contact, each gas will mingle with the other so thoroughly and automatically, that after a time, there is uniformity of composition of the gases contained in the several vessels. It has been ascertained that the relative velocities of diffusion of any two gases are inversely as the square roots of their densities. This diffusion is readily appreciated if it be borne in mind that gaseous matter is to be regarded as an aggregation of molecules in which the attractive force which unites them is reduced to a minimum because the spaces they occupy are relatively great, and that these molecules are therefore in a constant state of rapid motion and bombardmenta state of things that is of course greatly enhanced by the application of heat. It is this bombardment that is the foundation of the pressure or elastic force exercised by gas confined in a vessel at any given temperature and pressure.

Comments are closed

GANGUE

The mineral matter enclosing or intimately associated with metallic and other ores.

Comments are closed

GUAIACOL CARBONTATE

2,C03, a white, crystalline powder soluble in alcohol and ether, is used as a nonirritating preparation in the treatment of phthisis, diarrhoea, and typhoid fever, acting as an antipyretic and antiseptic.

Comments are closed