GLASS G71 



several continental languages. Nevertheless, it cannot be doubted that a wide field of 

 improvement is open in this quarter, and that some more solid foundation is needed by 

 our manufacturers in this line, than the prestige of a name, or the force of capital. 



In France, as in England, the ingredients are mixed with some care, and intro- 

 duced into a crucible, heated by a powerful furnace. These ingredients are sand or 

 silica, carbonate of soda, and carbonate of lime, with perhaps a little ground felspar in 

 some cases. The carbonate of soda is first attacked by the silica, and its carbonic acid 

 . driven off, whilst the remaining silica and carbonate of lime become imbedded in the 

 vitrifying mass. As the heat increases, a more perfect fusion takes place ; and then 

 the carbonic acid of the carbonate of lime makes its way through the fused materials 

 by which they are mechanically mingled together during the effervescence, which is 

 technically termed the ' boil ; ' and, provided no after-separation ensues from the 

 process of ' settling,' the whole crucible or ' pot ' of glass will have a uniform com- 

 position. But, as we have seen, this depends altogether upon the relative proportion 

 of the materials towards each other, for an excess of either one or other of the bases 

 will destroy the homogeneous character of the whole, and introduce a plexus of striae. 

 Now the plate-glass of St. Gobain is almost exactly an atomic compound, and consists 

 of one atom of the trisilicate of soda and one atom of the trisilicate of lime, with a 

 small percentage of alumina. Science must ever take the lead of prejudice and 

 custom. 



The examination of English plate-glass fully corroborates the general result deduced 

 from the action of light. There is no approach to an atomic arrangement. The 

 principal constituent is trisilicate of soda, but variable quantities of lime, alumina, 

 and even magnesia, exist in it. Potash is sometimes present, and oxide of iron is 

 invariably so ; but in not one single instance, out of 17 samples examined with great 

 care, could so much as a surmise of the doctrine of combining proportions be gathered 

 from the result of the analyses. Similarly fruitless was a research instituted upon flint- 

 glass, both British and foreign. Of 35 samples analysed, no satisfactory evidence 

 could be adduced to favour the opinion that science had been a helpmate to industry, 

 or was at all concerned in this branch of manufacture. There are, however, some 

 points of vast interest associated with the practical working out of this matter. Potash 

 is known to give a more brilliant and harder glass than soda, and alumina seems to 

 tend in the same direction. The Bohemian glass, so celebrated throughout Europe, is 

 a glass of this description, and contains silicate of alumina, silicate of lime, and silicate 

 of potash, but not in chemical proportions. This glass is therefore striated, but it 

 seems to permit of a more perfect decoration by metallic oxides that can be de- 

 veloped in glass of lime and soda. This very probably depends upon the alumina 

 contained in it. From one singular oversight, the use of carbonate of baryta has not 

 yet found its way into the composition of glass, though we can scarcely conceive a 

 more hopeful material. This substance may be had in large quantity in the North of 

 England, of great purity, and at a merely nominal cost as compared with its value 

 for such a purpose as glass-making. That it would fuse readily with a due amount 

 of soda, and give ' a boil ' as well as chalk, there can be no doubt ; whilst its great 

 density will certainly improve the refractive power of the resulting product, and thus 

 rival the brilliancy of lead- or flint-glass, without imparting that softness and liability 

 to receive scratches which are so objectionable in the la'tter variety. One difficulty 

 may perhaps reside in the want of information concerning the quantity to be em- 

 ployed. But this is easily adjusted ; for it has been demonstrated that, during vitri- 

 fication, the silicic acid unites to bases in the proportion of three atoms to one : 

 consequently three atoms, or 138 parts, will always require one atom of each base. 

 Therefore, this weight of good dry sand may be set against 54 of dry carbonate of 

 soda, 70 of carbonate of potash, 50 of pure marble or chalk, 99 of carbonate of baryta, 

 and 112 of oxide of lead or litharge. Suppose, then, that the object is to employ 

 carbonate of baryta for the first time, here 6 atoms or 276 parts of sand, 1 atom or 54 

 parts of dry carbonate of soda, and 1 atom or 99 parts of carbonate of baryta, may be 

 mixed and fused together with every prospect of obtaining a good result ; or 9 atoms 

 of silica, 1 of carbonate of potash, 1 of carbonate of soda, and 1 of carbonate of baryta, 

 might be tried without fear of failure. Again, in the case of flint-glass, 112 of 

 litharge, 54 of soda, and 276 of sand, would probably succeed, or an additional atom 

 of trisilicate of potash might be used. For many years past, M. Dumas, now, perhaps, 

 the first chemist in France, has been in the habit of demonstrating to his pupils that 

 glass of all kinds, when properly made, must necessarily be an atomic compound ; 

 and yet we scarcely expect to find a single British glassmaker who will admit that his 

 art is susceptible of such decisive and beautiful simplification. 



To assist as far as we can in the attainment of this end, we shall proceed to describe 

 a simple means for the analysis of glass, which will enable any person, possessed of 

 even very trifling chemical skill, to determine the composition of any given sample of 



