244 PHENOMENA DEPENDENT ON MOLECULAR PATHS 91 



named ; it consists in the introduction of a law of dependence 

 of the density on the pressure, which embraces both the 

 liquid and the gaseous states of the substance. A second 

 possibility of improving the theory lies in the consideration 

 that, in addition to single molecules C0 2 and double ones 

 C 2 4 in the gas, there may also be present triple molecules 

 C 3 6 , quadruple ones C 4 O 8 , &c. 



The want of the necessary leisure alone prevents me 

 from carrying out these calculations. 



92. Transition into the Critical State 



In the combination of the simple molecules of a gas or 

 vapour to form larger masses we must doubtless see an 

 approximation to the liquid state in which all particles 

 are joined together into one cohering mass. If, now, the 

 viscosity increases when the density rises by aggregation of 

 the molecules, as in the theories and observations we have 

 before discussed, we shall have to conclude that the viscosity 

 of a vapour attains its greatest value when the vapour has 

 attained the saturated state. 



But an experiment of Lothar Meyer's, 1 on the vis- 

 cosity of the vapour of benzol, seems to contradict this. He 

 allowed saturated benzol vapour to pass through a capillary 

 tube into a space where the pressure was less ; the vapour was 

 here condensed by cooling, and the mass of vapour which 

 had traversed the capillary tube in a given time was deter- 

 mined by weighing the liquid. From this weight the 

 coefficient of viscosity of the vapour was calculated by 

 Poiseuille's law. Calculation gave the value of this co- 

 efficient (which is constant for gases) to be the smaller the 

 higher the back pressure at the exit of the capillary tube. 

 We may therefore also say that the friction seemed to be 

 the smaller the larger the mean pressure was in the tube ; 

 but this is the exact opposite of the theoretical conclusion, 

 that the viscosity of a vapour is the more considerable the 

 nearer the vapour is to the saturated state. 



Lothar Meyer has explained this apparent contradic- 



1 Wied. Ann. 1879, vii. p. 531. 



