528 Mr. W. A. Shenstone [March 8, 



The Properties and Applications of Vitrified Silioa. 



Vitrified silica is harder than felspar, but less hard than chalce- 

 dony. When cut with a file it can be broken like glass. Its conducting 

 power for heat is about equal to that of glass. Mr. Boys has shown that, 

 even in an atmosphere saturated with moisture it is a very good insu- 

 lator. Its density* (2*21) is decidedly less than that of quartz (2 '66), 

 and very nearly as low as that of ordinary amorphous silica. Its optical 

 properties have not yet been fully studied, but its approximate index 

 of refraction has been determined by Professor S. P. Thompson, by 

 means of a small prism cut for the purpose by Mr. Hilger ; it is 

 decidedly less than that of quartz. 



The melting-point of silica is not known, and it is plastic over a 

 considerable range of temperature. When a platinum wire embedded 

 in a thick tube of silica is heated from outside by means of an oxy- 

 gas flame the platinum melts and runs at a temperature at which the 

 silica retains its shape. 



The rate of expansion of vitreous silica has been studied by 

 H. Le Chatelier,f and recently by Professor Callendar. The former 

 finds its mean co-efficient of expansion between 0° and 1000° to be 

 0*0000007, but from the manner in which his material was prepared 

 I think it probable that it was not quite pure. Professor Callendar 

 has, within the last few days, examined the behaviour of a rod of pure 

 vitrified silica prepared for him by my method. He finds its mean 

 co-efficient of expansion to be • 00000059, which is only yL as great as 

 that of platinum, and much smaller than that of any similar substance 

 that has hitherto been studied. He finds also tbat the expansion of 

 vitrified silica is exceedingly regular up to 1000°, and that if not 

 heated above 1000° the rod returns very exactly to its original 

 length when cold. Beyond 1000° he found a slight permanent elon- 

 gation, although the rod was under compression. Professor Callendar 

 was able to carry his experiments up to 1500°, which is very satisfac- 

 tory, for it shows that vitrified silica remains solid, or practically 

 solid, at this very high temperature ; this is an important observa- 

 tion, as less carefully conducted experiments made by others had led 

 me to fear that the substance might be found to become slightly 

 plastic even at as low a temperature as 1000°. Above 1000° the rate 

 of expansion diminishes rapidly, changing to a contraction at about 

 1200°4 On cooling from 1500° to 1200° it expands. 



Fine rods of silica and also quartz fibres are apt to become 

 rather brittle after being heated to redness. But we have not at 

 present detected this defect in the case of thick tubes or rods. 



The transparency of vitrified silica to ultra-violet rays has been 



* This was determined by my pupil, Mr. T. Pears, the silica used contained 

 a few minute bubbles. 



t Comptes Eendus, cxxx. 1703. 



I H. Le Chatelier's curve, see Fig 5, shows a similar contraction, but 

 commencing at a somewhat lower temperature. 



