EXPERIMENTAL KNOWLEDGE OF THE PROPERTIES OF MATTER. 103 



The principle of the measurement of pressures was the same, therefore, 

 as that used by Regnanlt. 



Yarious devices have been used by Cailletet for applying and measur- 

 ing increased pressures ; and Amagat suggests a plan which might be 

 adopted, and by which the experiments might be extended to vastly in- 

 creased pressures — namely, having, e.g., detei'mined the law of compres- 

 sibility by a vertical height of TQ metres of mercury up to 100 atmos 

 pressure, we may extend it up to 200 atmos by exerting on the top of the 

 column pressures up to 100 atmos. But Amagat prefers the direct 

 method, and applies it up to 4B0 atmos ; though for the purpose of ex- 

 tending the results to still higher pressures he has ' all the apparatus 

 necessary. 



One most striking fact which results from A magat's observations — and 

 it is so uniformly true for the gases which he tised and for all the tempera- 

 tures at which he experimented, and whether the gas was or was not 

 liquefied in the course of compression, that it has the appearance of being a 

 natural law — is that for each gas beyond a certain pressure the law of 

 compressibility is more and more nearly represented by the equation — 



V (v—a)=b, 



where a and h are constant for a given substance at a given temperature ; 

 and o varies but slightly for the same substances at different tempera- 

 tures. 



The curves by which the results are represented ^ have for abscissae 

 the values of p in metres, and for ordinates the values of ^ji? ; and for 

 above 180 metres the curve becomes almost straight. 



In the case of the gases liquefied (as, e.g., COo at 18°, one of the tem- 

 peratures of the experiments), as also of gases which, though not lique- 

 fied, were compressed near their critical points (e.g., ethylene), the 

 only regular part of the curve is that at high pressure, which is nearly 

 straight. 



For hydrogen the relation between pv and p is represented on the 

 diagram by a straight line for all except the very lowest pressures. The 

 temperatures for which Amagat obtained curves of relation of vp to p 

 were temperatures at intervals between about 15° and 100°, giving from 

 4 to 10 curves for each gas. 



So far we have said nothing of experiments made as to relation of 

 volume to pressure where the pressures are less than 760 mm., and espe- 

 cially when they are very small. Special investigations with this object 

 were undertaken by Siljestrom (' Pog. Ann.' 1874) ; by Mendeleeff with 

 others ('Ann. Cliim.' (5) ii. 1874, and (5) ix. 1876; also 'Nature,' 

 X. 1876-7) ; and by Amagat (' Ann. Chim.' (5) viii. 1876 ; and (5) xxviii. 

 1883). 



Experiments on Belation between Pressure and Volume of Gases at 

 Pressures below 1 Atmo. 



Mendeleeff and Siljestrom come to the conclusion that for small pres- 

 sures the compressibility is less than Boyle's law requires, and that the 

 limiting condition of a gas is that in which the density is nil and the 

 pressure finite ('Ann. de Chim. et de Phys.' 6, xxviii. p. 482). 



' Annates de Chimie et de Physique (5), xix. 1880, p. 348. 

 2 Ibid. (5), 1881, p. 22 



