According to Dalton, the particles of one gas possess no repulsive or attractive power with regard to the particles of another gas; and accordingly, if
m
measures of a gas A be mixed with
n
measures of another gas B, each will occupy
m
+
n
measures of space. The density of A in such a mixture will be
m
/
m
+
n'
and of B,
n
/
m
+
n'
, the pressure upon any one particle of such a gaseous mixture arising solely from particles of its own kind. “It is scarcely necessary,” Dalton remarks, “to insist upon the application of this hypothesis to the solution of all our difficulties respecting the constitution of mixed gases where no chemical union ensues. The moment we admit it every difficulty vanishes. The atmosphere, or, to speak more properly, the compound of atmospheres, may exist together in the most intimate mixture without any regard to their specific gravities, and without any pressure upon one another. Oxygen gas, azotic gas, hydrogenous gas, carbonic acid gas, aqueous vapour, and probably several other elastic fluids, may exist in company under any pressure, and at any temperature, without any regard to their specific gravities, and without any pressure upon one another, while each of them, however paradoxical it may appear, occupies the whole space allotted to them all.” In conformity with this law, Gay Lussac found that the vapours of alcohol and water mix like two gases which have no action upon one another. The density of the mixed vapours agreed closely with the density calculated according to Dalton’s law. In 1836 Magnus published an important memoir on the same subject. He found that, if two liquids which do not mix with one another are introduced into a barometer tube, the tension of the mixed vapours at any temperature is equal to the sum of the tensions of the vapours of the two liquids. But when the liquids have the property of mixing with one another the behaviour of their vapours he found to be altogether different. The tension of the mixed vapours was no longer equal to the sum of the tensions of each vapour separately. This statement appears at first view to contradict the experiments of Gay Lussac, but, as Magnus himself has pointed out, the conditions under which the observations of the two eminent physicists were made were essentially different. In the experiments of Gay Lussac the mixed liquids were wholly converted into vapour, and therefore the mixed vapours formed were not in contact with any liquid, while in those of Magnus an excess of the mixed liquids was always present, and in contact with the vapour,
ON THE CHANGE OF TEMPERATURE CONSEQUENT ON ANY CHANGE IN THE DENSITY OF ELASTIC FLUIDS, CONSIDERED ESPECIALLY WITH REFERENCE TO STEAM.
