(612.) 
Dalton’s 
writings. 
(613.) 
Researches 
on gases 
and va- 
pours. 
934 
his mind. In this he resembled Cavendish; but he 
did not resemble him, and in one sense may be said 
to have surpassed him, in the boldness, we might call 
it audacity, with which on a frequently too slender 
foundation of facts Dalton established, to his own 
satisfaction, by reasoning almost as much @ priori as 
that of the mathematician, comprehensive physical 
theorems expressing the laws of phenomena. 
The more important of these refer to the consti- 
tution of gases and vapours, together with their rela- 
tions to heat, and to the combinations of bodies by 
chemical affinity. We shall endeavour to give some 
MATHEMATICAL AND PHYSICAL SCIENCE. 
a 
[Diss. VI- 
Its elastic foree being added to that of the dry air, 
the whole will expand until equilibrium is restored 
with the constant pressure without, and this will oc- 
cur as soon as the elasticity of the dry air alone 
(proper to its increased yolume), added to the elasticity 
of the vapour alone (depending solely on its tempera- 
ture), are together equal to the pressure which they 
have to support. In short, to use the precise enun- 
ciation of Dalton himself, “in all cases the vapour 
rises to a certain force, according to temperature, 
and the air adjusts the equilibrium by expanding or 
contracting as may be required.’ 
The importance of this law (easily verified in the (614) 
particular case) is readily perceived. Not only did Theory of 
it affect the results of almost every experiment in rd es 
pneumatic chemistry, but it rendered a new theory of 
account of these researches separately. They are to 
be found detailed in the Memoirs of the Literary and 
Philosophical Society of Manchester (particularly the 
fifth volume), and in Dalton’s New System of Che- 
mical Philosophy, of which three successive parts ap- 
peared in 1808, 1810, and 1827, of which the first 
is the most original and important. 
I. We shall first speak of his researches connected 
with gases and vapours. It had been noticed by 
Priestley and others that when gases exercising ap- 
parently no chemical action upon one another, are 
mixed in a confined space, they become, after the 
lapse of a longer or shorter time, completely inter- 
mingled, and that without any regard to even great 
differences in their density, the particles of the lighter 
gas diffusing themselves contrary to gravity through 
those of the denser, and causing them in their turn to 
ascend. The uniform composition of our atmo- 
sphere, in all circumstances and at all heights, is a 
striking example of this property. In explanation of 
it, Dalton had in 1801 arrived at the conclusion, 
“ that the particles of one gas are not elastic or re- 
pulsive in regard to the particles of another gas, but 
only to the particles of their own kind.” The fun- 
damental experiment on which this singular con- 
clusion was based was the following:—That the 
quantity of vapour of water (which, when purely 
elastic, may be considered as a gas) which can exist 
uncondensed in a given space, depends solely upon 
the temperature, and is independent of the presence of 
air and of its own density. Thus water, or any other 
evaporable liquid, being introduced into a space con- 
taining air, or any other gas, of any density, but 
subject to a constant external pressure, the space in 
question becomes damp by the evaporation of the 
liquid ; now the amount of the latter converted into 
vapour depends, according to Dalton, upon the single 
circumstance that the vapour yielded by it must 
have the precise elasticity due to its temperature. 
hygrometry indispensable. The older theory of Hal- 
ley, Leroy, and Franklin was, that the direct affinity 
between air and water drew up a portion of the lat- 
ter into the former with the aid of heat, whilst De 
Saussure (Essai sur ’ Hygrométrie, 1783) believed 
that the conversion of water into vapour took place 
first and independently by the action of heat, and that 
it was then drawn up into the atmosphere by the at- 
traction of the gases which compose it. Dalton’s 
experiments show that the air and the vapour mix 
without the slightest mutual interference or reac- 
tion. He founded thereupon an excellent prac- 
tical method of determining the amount of vapour 
in the atmosphere. He first formed a table of the 
elasticities of watery vapour or steam for all tem- 
peratures between 32° and 212°; and so simple and 
accurate were his methods, at least for the lower de- 
grees of the thermometer, that his numbers are still 
received as amongst the best we have. He operated 
merely with a carefully constructed barometer, into 
the vacuum of which he introduced a few drops of 
water, and raising the temperature by means of a 
tube embracing it, and which could be filled with 
water at pleasure, he observed carefully the depres- 
sion of the mercurial column. The elasticities thus 
determined commenced with 0:2 inches of mercury at 
32° up to 30 inches at 212°. These numbers, con- 
sequently, represent the utmost elasticities of vapour 
which can exist either in air or without air at the cor-- 
responding temperature. If we attempt to add more 
vapour, or to lower the temperature, in either case 
moisture will be deposited. Hence to find the quan- plea 
tity of vapour in the atmosphere when not absolutely o,. 
damp, it is sufficient to ascertain the temperature at 
which it becomes so. This Dalton did by filling a 
1 The exact expression of the effect is this, wate where p represents the pressure expressed in inches of mercury upon a 
given volume (equal to unity) of dry air; f the force of the vapour in, vacuo at the temperature of experiment, also in inches of 
mercury ; and v the volume which the mixture of air and vapour occupies under the given pressure p after saturation. It is evident 
that p—f being the pressure due to the elasticity of the dry air apart from the vapour, when we affirm that the volume (1) becomes 
p - 
vapour, just as though vapour were absent or its space void. 
5 7 We in effect affirm that Mariotte’s (or Boyle’s) law connecting volume and pressure holds true for air which is mixed with 
a 
