286 Prof. Norton on Molecular Physics, 



stant, whatever may be the value of r. In fact the value of m 

 depends upon the pressure to which the molecular atmospheres 

 are exposed (vol. xxx. p. 99, foot-note) ; and this, in gases, must 

 depend mainly upon the barometric pressure, and be constant for 

 the same pressure. It is to be observed that the results given in 

 Table I. hold good for compound molecules, provided as they are 

 with their own especial atmospheres (/. c. p. 99), as well as for sim- 

 ple molecules. If r, the radius of the atmosphere of the mole- 



(7Yl\ 

 = -g ) will be dimi- 

 nished in proportion to the square of this ratio ; but if we esti- 

 mate the force at a given distance, this distance, as expressed in 

 terms of r, will be diminished in the same ratio that r is increased ; 

 and hence, if the force varies inversely as the square of the dis- 

 tance, its value at the given distance will be the same as before. 

 The elastic force of molecules posited at the same distance from 

 each other should therefore be the same, whatever may be the 

 radius r of the molecule, simple or compound. This result 

 depends upon the assumption that the force of molecular repul- 

 sion varies, beyond a certain limit, inversely as the square of the 

 distance. Table I. shows that for the smaller values of the ratio 



— (which should be taken for liquids that furnish vapours, and 



substances that habitually exist in the gaseous form) we have 

 nearly reached this limit at the distance 80r. 



To this it should be added that the calculations are made upon 



71 



the supposition that the ratio — remains constant for each set of 

 computations, answering to each special value of—. But as a 



matter of fact, in the transition from the liquid to the vaporous 

 state, as we have seen (vol.xxviii.p.282),the molecular atmospheres 



71 



expand, which should diminish the value of n, and of the ratio—. 

 r m 



Accordingly, to represent truly the vaporous state, it is probable 



that the calculations should be made for a smaller value of — 



m 



than any of those given at the head of the Table. If this be 

 true, the limit above referred to may be greatly reduced. This 

 should be the case especially with the permanent gases. The 

 distinction between the gases and vapours lies in all probability 



71 



in the fact of a smaller value of — for the former than the latter, 



m 



in consequence of which, compression or reduction of tempera- 



