audits Application to the Aqueous Vapour of the Atmosphere. 353 



vapour-pressure the mean reading of the barometer stands quite 

 as high as in a great vapour-pressure ; at the same time I con- 

 trived an easily-performed experiment, in which, contradictory 

 to Dalton's theory, a mass of vapour and a mass of air, placed 

 in communication with each other, mutually preserve a state of 

 equilibrium without the vapour penetrating into the air or the 

 air into the vapour. As the result of this, I laid down the 

 proposition that the vapour exerts a pressure upon the air and 

 the air upon the vapour ; and the atmosphere is to be regarded 

 as a mixture of masses of air, some more and some less humid. 

 Strachey furnished a second very solid proof of the inadmis- 

 sibility of Dalton's theory in a paper which he read before the 

 Royal Society of London in the year 1861. Proceeding upon 

 considerations which are fundamentally identical with those 

 developed by Bessel, he gave a collection of the results of ob- 

 servations which had been obtained upon high mountains and 

 in air-balloon expeditions, and showed that they were incom- 

 patible with the supposition of an independently-subsisting 

 atmosphere of vapour. To instance one point only, it may be 

 here mentioned that the observations of Welsh, who ascended 

 in a balloon to the height of 23,000 feet, place us in a position 

 to calculate the pressure which the vapour contained in the 

 atmosphere would exert on the earth's surface ; but the value 

 determined in this manner amounts only to the fourth part of 

 the pressure actually assigned by the psychrometer. It might 

 have been supposed, from the clearness of the proofs adduced 

 and the close agreement of all the results of observation, that a 

 finally satisfactory decision would have been arrived at ; never- 

 theless we find, even in the most recent times, that the " pressure 

 of the dry air " and the " pressure of the vapour-atmosphere " 

 are, as before, kept distinct the one from the other. There is, I 

 believe, no other means of removing the rooted ideas in con- 

 sequence of which " Dalton's laws " are constantly appealed to, 

 than the direct proof that Dalton's laws themselves contain an 

 essential error. 



With this view I undertook a short time since the series of 

 experiments to which I referred at the outset. I first convinced 

 myself how extremely slowly the vapour in the air spreads itself 

 from one part of space to another, if, without destroying the 

 communication between them, the free circulation of the air be 

 restrained. It is mainly the circulation of the air that carries off 

 the vapour from the evaporating surface, and conveys the vapour, 

 when already diffused, to the chloride of calcium to be absorbed ; 

 one would almost believe that the individual molecules of air 

 must come to the surface of the water to take thence the moisture, 

 and to the chloride of calcium to give up to it the moisture ; 



