310 DIRECT PROPERTIES OF MOLECULES 113 



that no other molecule can penetrate with its centroid into 

 them, as real spheres and calling them their spheres of 

 action. But, after the foregoing explanations, we must 

 hesitate to believe in the spherical form of the gaseous 

 molecules and, perhaps too, in the spherical form of their 

 spheres of action. 



Against this, however, it may be argued that the flat 

 discs which we call molecules are not at rest, but are con- 

 ceived as being continually in motion ; and since, too, they 

 are continuously turning round, they must exert their 

 actions equally in all directions of space, and we should 

 thereby be justified not only in calling the regions within 

 which their action is sensible their spheres of action, but 

 also in looking upon them as veritable spheres. 



But we have to consider that the surface conceived to 

 be constructed about a molecule obtains a somewhat dif- 

 ferent signification when it is assumed to be spherical. The 

 sphere of action has been enlarged to occupy a greater space, 

 which we may call the molecular volume ; for we may very 

 well so term that volume which a molecule at least 

 requires for itself. If the molecule were at rest, this space 

 would be the sphere of action, or that volume into which the 

 forces exerted by the molecule would not allow another to 

 penetrate; but the molecule is in motion, and requires, 

 therefore, a greater space. This will be smallest when the 

 molecule has no forward velocity and executes only rotatory 

 motions ; the rotation of the sphere of action then gives 

 rise to the molecular volume, or the space from which the 

 molecular forces strive to drive intruders now this way and 

 now that. The molecular volume is therefore the smallest 

 space required by the molecule in case it is not quite at 

 rest, or, in other words, robbed of its heat. 



What I have here called the molecular volume is not 

 essentially different from that which for many reasons has 

 been denoted by this term in theoretical chemistry. As is 

 well known, chemists call the molecular or specific volume 

 the volume measured in cubic centimetres of a mass which 

 in grams is numerically specified by the same number as 

 the molecular weight. It is therein assumed that the sub- 



