MEASUREMKNT OF HErGHTS. 535 



According to this \ne\v of our knowledge of vapour, no ground 

 is afforded for the hypothesis that vapour is compressed only 

 by vapour, and not by air ; and we lose at the same time the 

 analogy for the similar comportment of the mixture of different 

 gases. Dalton has adduced, in further support of his supposi- 

 tion, a circumstance which is independent of the experiments 

 on aqueous vapour, viz. that a specifically heavier gas mixes 

 with a lighter one, even though the latter should be placed 

 uppermost. It is true that Dalton's hypothesis explains this 

 fact; but it cannot be maintained that the fact is inconceivable 

 apart from the hypothesis. The ascent of fluids in tubes which 

 are wetted by them might, for example, be explained by the 

 assumption that gravity exerted its action but imperfectly 

 within the tubes ; but we know the true explanation is 

 different. If I do not mistake, the small amount of the altera- 

 tion which the constitution of the air undergoes in a space in 

 which there are many persons, whose breathing must diminish 

 the oxygen and increase the carbonic acid gas, has been ad- 

 duced in support of Dalton's views, as the oxygen must by pre- 

 ference replace itself from the outward air, and the carbonic 

 acid gas must pass to the same in preference, if the several con- 

 stituents of the interior air are compressed only by those of the 

 same nature without. The first experiments of the kind were 

 made by Humboldt and Gay Lussac in one of the Parisian thea- 

 tres*; and these gave a diminution of the oxygen of 0*007, with 

 an imperfectly determined content of cai'bonic acid gas, Daltou 

 subsequently repeated experiments of a similar kindf in spaces 

 filled with numerous assemblages, and found the oxygen = 

 0"20325, whereas in free air he found it 0*2090; there was also 

 more carbonic acid gas than in the free air, and in one case, in 

 which it was determined, the amount was 0"01. These experi- 

 ments do therefore show actual alterations in the constitution of 

 the air; and it only remains to examine whether they are less 

 than the alterations to be expected according to the older views. 

 The first-mentioned experiments do not appear to have been 

 made for the purpose of testing these ; and all are deficient in 

 the exact data requisite for founding a calculation; i. e. the 

 cubic contents of the room, the air of which was examined, — 

 the number of persons, and of the hghts, and the strength of 



• Gilbert's Ann. der Physik, vol. xx. p. 88. 

 + Phil. Trans. 1837, part II. p. 363. 



