46 
DR. T. ANDREWS ON THE PROPERTIES OE MATTER IN 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 condi¬ 
tions 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, t 
The same subject was afterwards investigated with great care by Regnault in his 
elaborate work on the elastic force of vapours. His experiments were made by a 
similar method to that adopted by Magnus, but they embrace a larger number of 
bodies, and the results are given in fuller detail. The conclusions at which Regnault 
arrived are the same as those previously stated by Magnus, viz., that two volatile 
liquids, which are not capable of dissolving each other, give a vapour tension equal to 
the sum of the tensions which the same liquids give separately, but that two volatile 
liquids, which are capable of dissolving each other, give a complex vapour whose 
tension is always less than the sum of the tensions of the vapours of the two liquids, 
and often less than the tension of the vapour of the more volatile liquid alone. J 
The case of the elastic force or tension of a mixture of gas and vapour has also been 
investigated by Regnault. In his earlier researches on this subject he found the 
elastic force of aqueous vapour, at the point of saturation, in presence of ah' or 
nitrogen, to be always a little feebler than its elastic force in vacuo. The difference 
was, however, small, rarely exceeding one-fiftieth of the whole tension, and Regnault 
in his earlier investigations was inclined to attribute it to some constant error in his 
method. § In his later researches he returned to the same subject, and in order to aid 
in the solution of the question he made a number of direct determinations of the 
density of the vapour of water within the limits of temperature at which he had 
formerly worked. He also extended his experiments to mixtures of air with vapours 
of other liquids more volatile than water. The results he obtained were in accordance 
with Dalton’s law, provided the mixed gas and vapour were compressed so as to 
* ‘ Annales de Chimie,’ vol. 95, 1815, p. 314; Biot, ‘ Traite de Physique,’ vol. 1, p. 298, where Gat 
Lussac’s experiments are first described. 
t ‘ Poggendorff, Annalen,’ vol. 38, 1836, p. 488. 
j ‘ Memoires de 1’Academic des Sciences,’ vol. 26, 1862, pp. 722 and 729. 
§ ‘Annales de Chimie,’ vol. 15, 1845, p. 137. 
