a 



35 i< Prof. Lamont on Dalton's Theory of Vapour, 



the expansive force of the vapour itself is in every case a matter 

 of small influence on its diffusion in the air. Now, if we 

 take a closed tube, A B C D, filled with air, and intro- j?i~ \ t 

 duce a small quantity of water through an aperture Ci — ii> 

 near A, which is afterwards immediately closed, into the 

 bottom A B of the tube, the water begins gradually 

 to evaporate, and the vapour ascends, after the expi- 

 ration of a certain time, up to a b. How then will the 

 pressure be distributed upon the interior sides of the 

 tube ? AL—JB 



If, as I have endeavoured to prove, by means of the above- 

 mentioned experiment, the vapour and the air exert a mutual 

 pressure upon one another, the expansive forces of the air and 

 of the vapour will act together in such a manner that an amount 

 equal to their sum will press upon all points of the interior wall ; 

 and if we take separately the pressure peculiar to the vapour 

 alone, it is precisely as great as if the mass of vapour was uni- 

 formly distributed in the whole space ABCD. A totally dif- 

 ferent state of things will result if the view set up by Dalton, 

 and generally accepted by philosophers, is well founded ; for as, 

 according to this view, the vapour diffuses itself in the interstices 

 of the molecules of air, without producing any mechanical effect 

 whatever upon the molecules themselves, no pressure at all can be 

 produced upon the interior side of the tube by the expansive 

 force of the vapour, under the circumstances indicated above ; 

 and no pressure takes place until the vapour reaches the upper 

 surface C D. 



The state of things here indicated is only a transitory one ; a 

 similar state may, however, be made permanent by maintaining 

 in the lower space ABab a higher, and in the upper space 

 a b C D a lower temperature. If we denote the lower space by 

 V, the upper by V, the lower temperature by t, the higher by t 1 , 

 and the corresponding expansive forces of the vapour by f(t) and 

 / (t ! ) ; also the expansive forces of the enclosed masses of air by 

 k(\+uf) and ^(l+a/') ; we have, according to the hypothesis 

 advocated by me, the expansive force of the mixture 



= vTv/ W 1 + "0 + SW + vTT' W1 + at ' ] +/W] 

 =*+ vSV< (Y ' + W) + VTT l(YM + Y ' m ^' 



whereas, according to Dalton's theory, the expansive force will 

 only amount to 



