116 REPORT— 1886. 



surface jnst balances the external actual or artificial atmospheric 

 pressure. This view is, in fact, the basis of all practical attempts to 

 measure boiling-points. 



Kahlbaum has developed another view, as will be seen. In order to 

 understand it, it is well to bear in mind circumstances, which are not un- 

 common, which tend to retard or prevent the ebullition and distillation 

 of a liquid, so that a liquid may sometimes be heated far above its 

 ordinary boiling-point without giving out vapour freely. The view 

 commonly taken is that these circumstances are excej^tional, in the sense 

 that they may be artificially exaggerated and that the obstacle opposed 

 to boiling is of a variable amount and that therefore no definite boiling- 

 points can be obtained while these circumstances exist; but that if 

 these obstacles can be removed then there is obtained the true boiling- 

 point at the pi'essure (say 7G0 mm.), which does not differ sensibly 

 from the temperature at which the vapour given off in vacuo exerts a 

 pressure of 760 mm. 



Kahlbaum,' in a memoire published at Leipzig, 1884, develops a theory 

 of ' specific remission ' (with which we are not concerned here), and in 

 connection with it gives an account of determinations of relations be- 

 tween temperature and vapour-pressure in which he asserts that the static 

 and dynamic conditions of a liquid necessarily give different boiling-points ; 

 and whereas Regnault said that the static method when it can be em- 

 ployed is always to be relied upon as giving trustworthy results, and always 

 to be preferred to the dynamic, Kahlbaum says the dynamic method alone 

 gives the true boiling-point. It is to be boi-ne in mind that Regnault 

 had found that in many cases in which the two curves — one by the 

 statical method and the other by the dynamical method — overlapped, they 

 coincided very nearly when the substances taken were pure. It is quite 

 clear, therefore, that the static and dynamic determinations cannot alwa3's 

 disagree as a matter of course, and on account of the necessity of over- 

 coming cohesion, &c. 



Kahlbaum ^ explains at some length his views as to the boiling-points 

 as found by the statical and the dynamical methods, and defines -^ boiling- 

 point thus: — 'I call by the name boiling-jDoint that temperature uf 

 the vapour of a liquid in agitation at which its molecules can , by their 

 united energy, overcome the collective attractions of neighbouring mole- 

 cules and the external pressure.' 



It cannot be doubted that, as in Dufour's and Gernez's experi- 

 ments on the retardation of the boiling-point of liquids in the absence of 

 air into which the liquid can evaporate, so in the cases mentioned by 

 Regnault, and in others similar, the temperature of liquid and of vapour 

 may rise in the dynamic method considerably above the temperature at 

 which the vapour-pressure in the static method is in equilibrium with the 

 atmospheric pressure ; that is, both liquid and vapour may be superheated. 

 But these are exceptional cases, and bodies usually boil under circum- 

 stances such that superheating may with care be avoided. Again, it is 

 easy enough by avoiding necessary precautions to heat the vapour above 

 the boiliag-point, and so to make the boiling-point of any liquid seem 

 higher than it is. 



' Berichte der BcuUchen CJiemiscken G. 1883, svi. II. p. 247C ; 1884. xvii. I. 

 p. 1245, and p. 1263 ; 1885, xviii. 

 2 Ihid. 1884, xvii. I. p. 1263. 

 ' Luc. cit. p. 1272. 



