536 GAS, AIR 



colours are rich, and the stono is froo from flaws, it constitutes a valuable gem, which 

 may bo distinguished by tho following properties : 



The colour should be blood or cherry-red ; on the one hand often mixed more or 

 If-s with blue, so as to present various shades of crimson, purple, and reddish violet, 

 ana on the other hand, with yellow, so as to form orange-red and hyacinth-brown. 



Tho stones vary in size from the smallest pieces that can bo worked to the size of 

 a nut. When above that size they^ are scarcely ever free from flaws, or sufficiently 

 transparent for the purposes of the jeweller. 



The garnets of commerce are procured from Bohemia, Ceylon, Pegu, and the Brazils. 

 By jewellers they are classed as Syrian, Bohemian, or Cingalese, rather from their 

 relative value and fineness, than with any reference to tho country from which they 

 are supposed to have been brought. 



Those most esteemed are called Syrian garnets, not because they come from Syria, 

 but after Syrian, the capital of Pegu, which city was formerly the chief mart for tho 

 finest garnets. The colour of the Syrian garnet is violet-purple, which, in some rare 

 instances, vies with that of the finest oriental amethyst ; but it may be distinguished 

 from tho latter by acquiring an orange tint by candle-light. The Syrian garnet may 

 be also distinguished from all tho other varieties of garnet in preserving its colour 

 (even when of considerable thickness and unassisted by foil), unmixed with the black 

 tint which usually obscures this gem. The Bohemian garnet is generally of a dull 

 poppy-red colour, with a very perceptible hyacinth-orange tint when held between 

 the eye and the light. When the colour is a full crimson it is called pyrope, or fire 

 garnet, a stone of considerable value when perfect and of largo size. 



The best manner of cutting the pyrope, is en cabochon, with one or two rows of small 

 facets round the girdle of the stone. The colour appears more or less black when 

 the stone is cut in steps, but when cut en cabochon, the points on which tho light falls 

 display a brilliant fire-red. 



Garnet is easily worked, and when facet-cut is nearly always (on account of the 

 depth of its colour) formed into thin tables, which are sometimes concave or hollowed 

 out.on the under side. Cut stones of this latter description, when skilfully set, with a 

 bright silver foil, have often been sold as rubies. 



The garnet may be distinguished from corundum or spinel by its duller colour. 

 Coarse garnets reduced to a fine powder are sometimes used as a substitute for emery 

 in polishing metals. 



Bohemian garnet. See PYROPE. H. W. B. 



GAH1TET, SOXiUBXiE (Grenat soluble}. See CARBOLIC ACID. 



GAS. (Gas, Fr. ; Gaz, Ger.) The generic name of all such elastic fluids as are 

 aeriform under a considerable pressure, at the zero of Fahrenheit. Oxygen, hydrogen, 

 and nitrogen, are permanent gases ; many of the other vaporiform bodies have been 

 condensed by the joint power of cold and mechanical force. See Watts's ' Dictionary of 

 Chemistry.' 



GAS, AIR. The idea of carburetting air, or a gas of low illuminating power, is 

 not by any means new, having been started almost at the same period that lighting by 

 coal-gas in the ordinary way was introduced ; but it was not until tho opening up and 

 working of the American oil-wells, by which a supply of very light petroleum oil or 

 spirit was obtainable, that any scheme in this direction had any chance of success. In 

 1862 a patent was taken out in France by a M. Mongruel, which proposed to ] 

 through an exceedingly light and inflammable spirit, tho air thus becoming charged 

 with a certain amount of a highly luminous hydrocarbon-vapour, in such a quantity 

 as to render tho mixture inflammable, and to allow of its being burned as an ordinary 

 combustible gas. This idea was introduced into England by a company known as the 

 Photogenic Gas Company, which received some amount of support at the time, but 

 from some cause or other did not prove a success. 



The various inventions brought into notice since that time for the purpose of 

 carburetting air and other gases have been based upon precisely the same principle, 

 but have differed considerably in tho apparatus employed. 



Many erroneous statements have been made on this matter for the want of scientific 

 knowledge ; for instance, in most cases the patentees themselves have declared that a 

 union takes place, other than mechanical, between tho gases of tho air and tho vapours 

 of certain hydrocarbons, which result in tho production of such gases as olofiant gas, 

 or acetylene ; an utter absurdity which would bo evident to anyone who would consider 

 under what circumstances such gaseous mixtures are produced. A vapour of any 

 hydrocarbon will, whether it exist by itself or whether it bo diffused through another 

 gas or gases, always bo such a vapour, and will invariably exhibit tho properties cha- 

 racteristic of it; and, if diffused through a gas, will be found condensiblo to tho 

 liquid from which it was formed, if proper means bo applied. It is a matter of 

 simple calculation from known data to ascertain the exact amount of hydrocarbon- 



