1901.] on Vitrified Quartz. 529 



carefully examined by Dr. A. Wynter Blyth, to whom I am greatly 

 indebted. His results may be gathered from the accompanying illus- 

 trations based upon his photographs. 



Fig. 2 gives the results of photographing the ultra-violet end of 

 the spectrum of the light produced by electric discharge between 

 poles made of an alloy of mercury, zinc, tin, and cadmium, after it 

 had passed through various kinds of glass, a sheet of quartz, and 

 air respectively. 



Fig. 3 gives a similar set of photographs in which the trans- 

 parency of air (1) is compared with that of plates of quartz, 3 and 

 4 mm. thick (2 and 3), and with plates of vitrified silica (4), soda 

 glass (5) and flint glass (6) of equal thickness, which were prepared 

 for the purpose by Mr. Hilger. It will be seen that vitreous silica, 

 like quartz, is as transparent to ultra-violet radiations as air itself. 



The fourth figure compares the light emitted by vacuum tubes 

 of vitrified silica (I.) and glass (II.) respectively. The bands shown 

 were obtained from tubes into which traces of hydrocyanic acid had 

 been admitted. In this case, as before, it will be seen that silica is 

 very superior to glass and practically equal to air in transparency. 



The most remarkable property of vitreous silica is its behaviour 

 under sudden changes of temperature. We have seen already that 

 tubes of it may be plunged suddenly into an oxy-gas flame without 

 injury, and I have mentioned the fact that apparatus made of silica 

 needs no annealing. But this is not all ; we may drop water on a 

 white-hot silica rod or plunge a white-hot silica tube into cold water or 

 even, by Professor Dewar's kind aid, into liquid air, without injuring 

 it in any way whatever, indeed, experiments seem to show that the 

 material gains very distinctly in regard to its elasticity after this 

 treatment. I need hardly point out how convenient tubes of such 

 a material will be to chemists or how many spoilt lecture experiments 

 may be avoided in future, by those who possess a silica tube. 



This last property of silica and the splintering of quartz find 

 an explanation in the results obtained by H. Le Chatelier * and by 

 Callendar. These observers, as already explained, have shown that 

 the rate of expansion of silica is exceedingly low, and moreover that at 

 temperatures much above 1000° it contracts when heated. Under these 

 circumstances it follows, first, that the strains set up in silica when 

 it is suddenly heated or cooled are comparatively small in amount, 

 and secondly that if, for example, vitrified silica be cooled from 

 1500° to below 1000°, the strains set up at the earlier stages of the 

 change will tend to neutralise those produced subsequently. These 

 facts enabled Le Chatelier to predict the indifference of vitrified silica 

 to sudden variations of temperature, but the actual phenomena had 

 been observed and exhibited in this country previously. 



The behaviour of quartz under changes of temperature is also 

 very peculiar. This was studied by Le Chatelier.f From his 

 curves which are given in Fig. 5, it may be seen that this form of 



* Comptes Rendus, cviii. 1046, and cxxx. 1703. t Ibid, oviii. 1046. 

 Vol. XVI. (No. 95.) 2 n 



