50 IDEAL AND ACTUAL GASES 113 



5O. Horstmann's Explanation 



A different explanation of the same circumstances given 

 by Hojrj^tinann 1 rests at bottom on the second of the 

 hypotheses described in 48, although Horstmann gives 

 it a quite different form. He throws doubt, in fact, on the 

 exactness of Avogadro's law in the case of vapours and 

 gases which do not strictly obey Boyle's law, and explains 

 it as being only approximately correct. This law is, on our 

 theory, a necessary consequence of the hypothesis that 

 pressure and temperature are caused by rectilinear motion of 

 the molecules alone. Hence Horstmann's assumption is 

 not essentially different from that discussed in 48, accord- 

 ing to which the to-and-fro straight paths of the molecules 

 are supposed to be joined together by curved parts. 



By this assumption also the observed anomalies can be 

 explained. The force of the blow with which a molecule 

 strikes against the wall of the space filled with gas or 

 vapour is greater if the molecules move in right lines free 

 from forces of cohesion than if they are drawn back from 

 the wall and into the interior of the space in curved paths 

 by the attraction of other molecules. The pressure, there- 

 fore, of a vapour or gas which contains a given number of 

 molecules will be the less the more frequently the molecules 

 are caused to move in curved patns, and the greater the 

 more frequently and the longer they move in straight paths. 

 If, therefore, the density is increased, the pressure increases, 

 not in the same ratio, but in a less degree, because the 

 increase of the number of collisions causes a diminution 

 of the force of a collision. On the other hand, by an in- 

 crease of temperature the pressure will increase in greater 

 measure, since the force of a blow increases not only for 

 the reasons already given, but also because the faster moving 

 molecules traverse longer straight paths. 



These views, therefore, also suffice to explain simply and 

 naturally not only the greater vapour-density possessed by 

 easily condensed gases and vapours in the neighbourhood of 



1 Ann. Chem. Pharm. Suppl. Bd. vi. 1868, p. 53 ; also in his Habilitations- 

 schrift, Heidelberg 1867. 



I 



