Preliminary Note on the Compressibility of Glass 359 



pendent of the hydraulic apparatus. The pressure is now 

 raised to the desired height, as indicated by the manometer, 

 and the ends of the rod are observed, and their position \vith 

 reference to the micrometer noted. The pressure is then 

 carefully relieved, and a displacement of both ends is seen to 

 take place, and its amplitude noted. The sum of the dis- 

 placements of the ends, regard being had to their signs, gives 

 the absolute expansion, in the direction of its length, of the 

 glass rod when the pressure at its surface is reduced by the 

 observed amount, and consequently also of the compression 

 when the process is reversed. As in the case of non-crystalline 

 bodies like glass there is no reason why a given pressure should 

 produce a greater effect in one direction than in another, we 

 may put the cubical compression at three times the linear 



traction for the same pressure. 



As yet I have only experimented on glass, and only on one 

 sort, namely, that made by Messrs Ford & Co., of Edinburgh. 

 It contains 56-29 per cent, of silica, 29-5 per cent, of oxide of 

 0-5 j pi T <ent. potash, 3-36 per cent, soda, 3 per cent. 

 alumina, and 1-05 per cent. lime. I have observed its com- 

 pressibility up to a pressure of 240 atmospheres, and before 

 proceeding to higher pressure I intrnd to determine the com- 

 pressibilities of other solids, especially metals, at pressures up 

 to 240 atmospheres. The reason for taking this course is, that 

 having got two glass tubes to stand this pressure, I am anxious 

 t-) utilise them as far as possible before risking them at higher 

 press n 



The pressure in these experiments was measured by a 

 manometer, whi h ennixt> -implv >f a mercurial thermometer 

 \sith a stout bull). \vhi h i^ immrrsed in the water under 

 pressure, whiNt its stem projects outside. The values of the 

 of this instrument wen- d -tei mined by comparing it 

 \\ith a piezometer < ontammu di-tilled water. This piezometer 

 been compared with others \\lu, h had been subjected to 

 the pressure of very considerable and nn-a-un-d >lumns of 

 water on the sound; I he mean apparent compressi- 



bility ot : >und to be 0*00004868 x ; or, 



>.*. Roy. Soc. Land., 1876, p. i 



