22 CONSTITUTION OF MIXED GASES. 



vapour enter a vacuum and an equal fpace previoufl y occu- 

 pied by a gas. No one would refufe its due importance to 

 this argument, after feeing or perufing a fatisfaclory demon- 

 ftration ; but I know of no fuch thing in Mr. Dalton's eflays. 

 He aflerts, indeed, that a quantity of dry air, confined in a 

 tube, receives an augmentation of force from the prefence of 

 water, equal to that which is produced by the fame caufe in 

 the exhaufied tube. But this is no demonftration, unlefs the 

 porofity of air be firft admitted ; for, without this conceflion, 

 Mr. D. has no right to eftimate the quantity of vapour prefent 

 by the fpring of it; this ought to be determined by weight 

 only. But when did Mr. D. weigh an exhaufted veflel, and 

 afterwards take its weight when filled with vapour? When 

 has he repeated the fame experiment upon the fame veflel 

 containing dry and moift air, and found the refult favourable 

 to his hypothefis ? Negative anfwers to thefe queftions mud: 

 convince an impartial judge, that the porofity of the air has 

 been again artfully introduced to eftablifh its own exiflenee : 

 thus we find the fundamental datum of the fyftem to be de- 

 raonftrated by arguing in a circle, which begins and ends with 

 the thing to be proved. 



On the other hand, Mr. D. is convinced that the admiffio» 

 of vapour into an open bottle expels part of the air. This is a 

 proof that fleam, fuppofing its exiflence in the atmofphere, 

 finds a fpace crowded with gafes lefs convenient than a va- 

 cuum ; it, therefore, makes room for itfelf, by diminifliing 

 the denfities of thefe gafes, and creates in them Mr. Dalton's 

 capacity for the reception of water. This fad being eftablifh- 

 ed, it will alfo be evident, that, when vapour is forced into 

 a gas confined in a clofe veflel, the former cannot make its 

 way through the latter, otherwife than by condenfing it : Such 

 is the nature of the pores of the air, according to Mr. D.'s 

 own principles. The atmofphere is alfo found to oppofe a 

 fimilar refiftance to newly developed gafes; for the gafeous 

 compound contained in an open veflel filled in part with a 

 fermenting liquor, is not a column of the atmofphere perme- 

 ated by an additional body of the carbonic acid, but a mixture, 

 in which the proportion of common air is comparatively fmall. 

 This fadt overturns Mr. D.'s general conclufion, that atmo- 

 fpheres may be added to the compound atmofphere at plea- 



fure, 



