352 IVof. Lamont on Dal ton's Theory of Vapour, 



in space void of air and space filled with air determined by 

 means of Dal ton's experiments : as to the mutual relations sub- 

 sisting between vapour and air, when they are simultaneously 

 present in the same space, the experiments afford no informa- 

 tion ; and this deficiency Dalton supplied by giving to the 

 second of the above-quoted propositions such an interpretation 

 as if no mutual relation whatever existed between vapour and 

 air, and as if they remained near each other without producing 

 any the slightest mechanical effect upon one another. It is 

 strange that physical philosophers have in general inconside- 

 rately accepted this theory, so important and so pregnant with 

 consequences, without remarking that it constitutes only a pos- 

 sible, but not a necessary result of the experiments. Not less 

 singular is it that meteorologists have treated Dalton's theory as 

 available for application to the aqueous vapour of the atmosphere, 

 and have supposed an atmosphere of vapour to exist independent 

 of the air, and sustained in equilibrium by itself alone, notwith- 

 standing that the third of the above propositions properly de- 

 clares that there is indeed always a tendency to a normal rela- 

 tion, which is conceived to be in the state of restoration, but 

 which is never reached, because, in consequence of the changes 

 constantly taking place, the requisite time to produce an equal- 

 ization is never afforded. 



Objections have from time to time been brought forward to 

 the existence of an atmosphere of vapour independently subsist- 

 ing. Bessel has (Ast. Nach. No. 236) adduced the consideration 

 that in such a vapour-atmosphere the expansive force of the 

 strata incumbent upon one another must diminish according to 

 a determinate proportion, but that, from different observations, 

 it may be concluded that this proportion does not really exist ; 

 his arguments, however, appear — principally, perhaps, owing to 

 the want of sufficient data from observation — to have produced 

 no impression ; nor was more notice taken of the experiments of 

 Broun in Makerstoun (Report to Sir T. Brisbane) and Jelinek 

 in Prague (Denkschriften der Wiener Akad. math.-naturw. Classe, 

 vol. ii.), who proved by experiments that in different localities 

 situated very near together, where the same reading of the baro- 

 meter is observed, a very different vapour-pressure may be indi- 

 cated. One of the most zealous opponents of Dalton's theory 

 was Espy, who (especially in his second Report on Meteorology) 

 exposed its defects with much penetration, without, however, 

 furnishing a precise refutation. I believe that I have myself 

 brought forward the first proof of the incorrectness of the 

 theory {Denkschriften der Munclmer Akad. math.-phys. Classe, 

 vol. viii.) in the year 1857, when I showed, by means of ob- 

 servations extending through many years, that in a small 



