404 



GLASS. 



louze's conclusion, to the effect that in the trans- 

 formation from transparent to devitrified glass 

 there was no change of composition or weight. 



The results of Dr. WetheriU's experiments, 

 though presenting some points of difference, 

 still confirm Leydolt's view, as to a crystalline 

 structure in ordinary glass. Admitting the 

 presence of deposited crystals in some cases, 

 and the difficulty or impossibility of deciding 

 in certain others, the author's conclusions are 

 that the window-glass examined contained 

 crystals already formed, of which some are 

 more soluble than their matrix in fluorhydric 

 acid, and other perhaps less so ; and that all 

 the specimens of glass submitted to the action 

 of fluorhydric acid yielded crystalline forms. 

 Some of the observations appeared to show 

 that crystals formed at first during the refrig- 

 eration of the glass, were subsequently broken 

 by the operations of pressing, rolling, etc., to 

 which the material had been subjected. 



Solubility and Erosion of Glass. M. Pelouze 

 showed, some years ago, that when window, bot- 

 tle, and other varieties of glass are reduced to 

 fine powder and mixed with water, they are 

 soon acted on, yielding often as much as 8 or 

 10 per cent, of their weight to the water if 

 cold, and by boiling in water sometimes as 

 much as 36 per cent. the matter so separated 

 being in many cases a definite silicate of soda. 

 That powdered glass should so dissolve in 

 water, while yet glass in large masses or in 

 plates is in comparison so little acted on, the 

 author explained by differences, in the respects 

 of closeness, extent, and permanence of the 

 liquid contact, in the two sorts of cases. 



Dr. F. 0. Calvert opposes to the hypothesis 

 just named the fact that aqueous liquids, such 

 as wine, etc., appear to exert very little action 

 upon the internal surface of a bottle, though 

 they may remain in contact with it for many 

 years ; and he suggests that a peculiar molecular 

 condition of the surface of the glass, arising 

 perhaps under the circumstances of its manu- 

 facture into plates or otherwise, or possibly 

 from the atmospheric pressure, may be influen- 

 tial in enabling it to resist the action of water 

 and aqueous liquids. He cites the apparently 

 parallel facts, that the surface of polished steel, 

 the rolled surface of wrought iron, and the skin 

 of cast iron, resist the chemical action of both 

 air and acids in a far greater degree than does 

 the interior substance of the same masses. 

 Still, admitting this view, there are many facts 

 which go to show that the immunity of any 

 glass and in any part from chemical action is 

 comparative only, not absolute, and that differ- 

 ent samples of glass possess in very different 

 degrees the power of resisting such action. 

 These conclusions have recently received new 

 confirmation from the observations of M. 

 Weber, now to be referred to. 



Efflorescent and Deliquescent Sorts of Glass. 

 A paper on this subject, by M. R. Weber, 

 appears in Le Technologiste for August, 1864. 

 It has long been observed that many sorts of 



glass, especially when exposed to a moist air, 

 undergo with time changes at their surface, 

 their brightness and transparency being im- 

 paired, and the surface becoming covered with 

 a thin iridescent layer, or even yielding minute 

 scales, or showing cracks and fissures. Some 

 sorts of glass show these changes in a high de- 

 gree ; but of other sorts the surface and trans- 

 parency remain unimpaired for great lengths 

 of time, and though little care is taken of them. 

 The changes here referred to depend primarily 

 and chiefly on a slow decomposition of the 

 glass, under the influence of carbonic acid and 

 other atmospheric agents ; but, once set up on 

 its surface, their effect is increased, and their 

 occurrence often indicated, by the presence of 

 moisture on the parts and the firmer adhesion 

 of dust upon them. 



That the more or less rapid decomposition 

 of different sorts of glass is determined chiefly 

 by their chemical composition, is placed be- 

 yond a doubt by such facts as that the ordinary 

 soluble glass, which dissolves completely in 4 

 or 5 times its weight of boiling water, is es- 

 sentially a compound of sand and potash, in 

 about the proportions of 2 to 1 ; and that, in 

 various glasses of commerce, the proportion of 

 silex may vary from 45 up to 71 per cent, of 

 the whole, that of the alkali from 3 to 22, and 

 that of the lime from about 9 to 29 per cent. 

 When deposits form on the surface of glass, and 

 from its material, they have an alkaline reac- 

 tion, and reveal alkalies also by spectrum analy- 

 sis. The efflorescent product chiefly contains 

 soda ; the deliquescent, potash ; and it is al- 

 ready safe to assign an undue proportion of 

 alkali in its composition as a chief cause of the 

 easy corrosion and poor quality of glass of 

 certain sorts. Of course, glass thus defective 

 in quality is little suited for use in windows or 

 for mirrors ; while for optical purposes it must 

 be rejected. 



MM. Vogel and Reischauer proposed to de- 

 tect poor qualities of glass by exposing samples 

 for several days to the action of a heated and 

 concentrated solution of nitrate of zinc ; but 

 this and other proposed tests have the defect 

 that the testing liquid removes more or less 

 completely the product of its own action, and 

 that, though the surface of the glass be actually 

 attacked, the effect is not always evident on 

 subsequent inspection. M. Weber accordingly 

 adopted the plan (more fully described in his 

 paper) of acting on a glass to be tested, in 

 plates or other convenient form, with the fumes 

 of concentrated chlorhydric acid. Having first 

 cleansed the glass with great care, he exposes it 

 during twenty-four to thirty hours to the acid 

 fumes, and then shuts it up within a closet for 

 twenty-four hours longer. Next carefully re- 

 moving from the samples any ammoniacal va- 

 pors and dust, he examines the glass, first by 

 transmitted light, and afterward, if necessary, 

 by reflected light of oblique incidence. If, ir 

 the former manner, a whitish crust or deposit 

 is detected on the glass, and which is thin, and 



