228 



Profs. G. D. Liveing and J. Dewar. [June 6 y 



molecules are subject during their encounters with one another. 

 Increase of temperature would affect the former, tending to diminish 

 the number of complex molecules formed at a given pressure, but 

 would have no effect on the latter, for though the number of 

 encounters of the molecules in a given interval of time would be 

 greater the higher the temperature, yet so long as the volume was. 

 unaltered the ratio of the duration of an encounter to that of free 

 motion would be sensibly unaltered. So far as any change due to 

 temperature has been observed, it is that a rise of temperature slightly 

 weakens the diffuse absorptions. 



Tie verting to the compounds of oxygen, none of them show the ab- 

 sorptions of oxygen, not even the general absorption of the ultra-violet 

 rays. Some of them, such as water and carbon dioxide, appear quite 

 transparent to ultra-violet rays, while in others, such as nitrous oxide, 

 which show a general absorption of the ultra-violet rays, the limit of 

 transparency is different from that of oxygen. In other respects we 

 may say that there is no resemblance between the absorptions of the 

 compounds of oxygen and those due to oxygen itself. Some of the 

 former have very definite and characteristic absorptions, such as the 

 well-known spectra of the peroxides of nitrogen and chlorine, and 

 we must regard these as indicating the rates of vibration which the 

 molecules NO 2 and CIO 2 respectively are capable of easily taking up. 

 The absence of the absorptions due to oxygen from all compounds of 

 oxygen seems to indicate either that chemical combination is not, as- 

 has been supposed by some chemists, a temporary relation in which 

 the molecular groupings are continually breaking up, to be formed 

 anew with ever-changing elementary atoms ; or, that the periods of 

 dissociation are very small compared with the periods of association. 

 For otherwise we should expect that such compounds of oxygen as 

 CO 2 and N~0 2 must always have amongst their molecules some mole- 

 cules identical with those of oxygen and capable of taking up vibra- 

 tions of the same period. At least we must conclude that little, if 

 any, of the oxygen of these and other compounds is ever out of the 

 influence of the other components. 



We have re-examined the absorption-spectrum of N 2 4 at various 

 temperatures, and agree to the conclusion of Bell ('Amer. Chem. 

 Journ.,' vol. 7, p. 32) that N 2 4 , whether liquid or gaseous, effects 

 only a general absorption at either end of the spectrum, and that the 

 selective absorptions observed with it are due to the presence of 

 NO 2 . 



In order to obtain pure N 2 4 , the tube in which the liquid was 

 sealed was placed in a freezing mixture, and a large part of the 

 liquid frozen ; the remaining liquid was then drained as completely 

 as possible into the other end of the tube, and sealed off. 



It should be observed that the crystals of K 2 4 appear colourless, 



