liO On the Theory of mioced GaSes. 



suiting from the general action of the mass, not by a ftreg- 

 sure occasioned by a particular member of it. 



On this account it is in)possibIc for the aqueous part of 

 common air to preserve the character of a gas at low tempe- 

 ratures, because steam cannot support 30 niches of mercury 

 unless it is heated to 212 degrees of Fahrenheit's thermo- 

 meter: were it then practicable to n)ix vapour of a less heat 

 with atmospherical air, the spring of the gases would reduce 

 it in an instant to the state of a liquid ; so that the difficulty 

 which renders De Luc's theorv objectionable in its original 

 form, is not removed in reality by the present modification 

 of it. 



The theory of mixed gases has been found to be indefensi- 

 ble on the principles of the mechanical philosophy; and I 

 suspect that part of it which relates to the separate existence 

 of vapour in the atmosphere, will prove equally unfortunate 

 when brought to the test of experiment. Mr. Dalton, in 

 all probability, supposed he had done all that the confirma- 

 tion of this theory required, by inventing the doctrine of 

 separate equilibria ; for nothing more has been offered in 

 support of his opinions, particularly of that relating to the 

 existence of uncombined vapour pervading the atmosphere, 

 unless the statement of the following experiment, with his 

 explanation of it, may be referred to this head. If two par- 

 cels of dry air, which are equal in bulk, density, and tem- 

 perature, be confined by equal columns of mercury, in two 

 tubes of equal bores, one of which is wet and the other dry, 

 the air, which is thus expo':ed to water, will expand more 

 than that which is kept dry, provided the corresponding 

 augmentations of their temperatures be equal; which phae- 

 nomenon is thus explained on the principles of the theory : 

 The vapour that arises from the sides of the wet tube, pos- 

 sesses a spring of its own ; therefore it takes off part of the 

 weight of the mercury from the air, and thereby leaves it to 

 expand itself, so as to re-adjust the equilibrium. According 

 to this explanation, if I and g represent the lengths of the 

 columns of dry and moist air at any temperature; and if c 

 denote the length of a column of mercury, equal in weight 

 8 to 



