Water in Hot Glass. 467 



recognized with certainty. No observations were possible 

 until after 30 minutes' boiling. 



In the absence of systematic data, however, certain inci- 

 dental results of interest were obtained with this tube, by 

 observing at high and low temperatures alternately. Thus, 

 after about an hour's boiling, /3 fell off from 120/10 6 at 185° 

 to about 40/10 6 at ordinary temperatures. After about 

 2i hours' boiling, £ fell off from about 230/10 6 at 185° to 

 90/10 6 at ordinary temperatures. After about four hours' 

 boiling the hot compressibilities reached 400/10 6 ; but the 

 tube broke before the corresponding cold compressibilities 

 were measured. 



Opportunity was also afforded for roughly measuring the 

 coefficients of expansion of water-glass. After two and a 

 half hours' boiling the mean coefficient per degree was 

 about '002, between 25° and 185° 0. The corresponding 

 mean coefficient for pure water, according to Mendeleef *, is 

 •0008 to -0009. 



Thus it appears that even the relative contraction of water- 

 glass is over twice as large as the normal value for water 

 under like conditions. After four hours' boiling another 

 measurement of the coefficient of expansion of water-glass 

 gave nearly the same result as before. This coefficient did 

 not, therefore, keep pace with the increase of j3 ; but, as only 

 a small part of the thermal contraction on cooling lies in the 

 region of high temperature to which /3 refers, the two 

 coefficients should not keep pace. 



Finally, a comparison between /3 and volume-contraction 

 in the lapse of time showed a change of /9 of about 18/10 G 

 for each per cent, of volume-contraction relative to the cold 

 volume at 20° C. This mean rate is more rapid than the 

 rate observed for tubes 1 and 2, or the following tubes. 



5. The next tube examined was No. 4, with the following 

 constants : — 



Temperature of Vapour-bath, 210°. 



Internal diameter (cm.) 



Section. 





Original -0240 



•00045 



Cold thread-length 

 15-43 cm. at 20°. 



Corroded .... -0710 



•00396 



External diameter, 

 •55 cm. 



A higher temperature of exposure (210°) was here chosen 

 for comparison, in order to bring the corrosive action of the 



* Phil. Mag. (5) xxxiii. p. 108 (1892). . 



