(615.) 
Laws of 
mixed 
gases, 
(616.) 
Dalton’s 
Cuap. VIL, § 3.] 
thin glass with cold spring water, or by adding, if 
necessary, the solid nitrate of ammonia, until the 
temperature fell so far that dew began to be deposited 
on the surface. This excellent method constitutes 
in fact the dew-point hygrometer.. It has received 
various forms from Mr Daniell and others, but the 
original one is probably the best. 
These important laws and deductions being fully 
understood, the theory of the mixed dry gases follows 
as asimple corollary, For it merely asserts of them 
what has been admitted in the case of steam, that 
they may diffuse themselves, and exert their elastic 
forces quite independently of one another; so that, for 
example, in our atmosphere the total pressure is made 
up of the partial elasticities of the oxygen, nitrogen, 
and carbonic acid which compose it, each acting to 
the same emount as if it alone had existed in the 
space which it occupies. In his essay of 1801, Dal- 
ton stated his view of these facts thus :—‘ The par- 
ticles of one elastic fluid may possess no repulsion or 
attractive power, or be perfectly inelastic with regard 
to the particles of another,” 
Discoveries so practical could not fail to excite 
immediate attention, especially at a time when the 
mechanical researches of chemists were earnestly directed towards 
ideas on 
the subje 
ct, the gases. 
“The facts and experiments,” Dalton 
tells us in his Chemical Philosophy, ‘were highly 
valued ; some of the latter were repeated and found 
correct, and none of the results controverted ; but 
the theory was almost universally misunderstood.” 
Its opponents were Berthollet, Thomson, Henry, and 
others, and the replies of the author are contained in 
the work just cited. But he appears to have felt the 
force of the objection, ‘« Can it be conceived that an 
elastic substance exists which adds its volume to that 
of another, and which, nevertheless, does not act on 
it by its expansive force ?”—for in his chemical phi- 
losophy he abandons the comparison of gaseous par- 
ticles to similar magnetic poles which repel each 
other, but are inert towards non-magnetic matter, and 
allowing that heat is the primary cause of repulsion 
in all gases, ascribes their diffusion contrary to gravity 
to the dissimilar size of their spherical molecules. 
‘‘ The particles of one kind being from their size un- 
able to apply properly to the other, no equilibrium 
can ever take place amongst the heterogeneous mole- 
cules. The intestine motion,” he adds, “ must there- 
fore continue till the particles arrive at the opposite 
surface of the vessel against any point of which they 
can rest with stability, and the equlibrium at length 
is acquired, when each gas is uniformly diffused 
through the other.” It may be seriously doubted 
whether this theory of the facts will bear examina- 
tion, at least no attempt has been made to demon- 
strate it on mechanical principles. The subject has 
been allowed to remain during more than forty years of 
unequalled activity in such speculations without ma- 
terial light being thrown upon the proximate causes 
HEAT (ATOMIC CHEMISTRY).—DALTON. 
935 
of these wonderfully general and simple truths. Yet 
it can hardly be doubted that the mechanical theory 
of the gases and vapours is capable of a great exten- 
sion, and even of being illustrated by simple experi- 
ments. But the attention of chemists has been with- 
drawn from the physical bearings of their science by 
the prodigious increase in the number of compounds 
which they have had to analyze and classify. The 
most important sequel to Dalton’s discoveries has 
probably been that of Professor Graham, whose ex- 
periments prove that gases when separated by a 
porous partition permeate it in both directions, until 
they have mixed in proportions which are inversely 
as the square root of their density. This law clearly 
shows the purely mechanical causes by which the 
diffusion is effected. 
A discovery of Dalton, which has scarcely been 
considered second in importance to those we have 
mentioned, is that the rate of expansion of all gases 
by heat is the same. Thus thermometers of air, hydro- 
gen, and carbonic acid would all mark thesame degree 
when plunged in the same medium; whilst the mer- 
curial thermometer shows a more rapid expansion, at 
higher temperatures, if the air thermometer be taken 
as the standard. ‘This important fact was soon 
after but independently announced by Gay-Lussac of 
Paris. Dalton’s publication dates from 1801, In 
his Chemical Philosophy, he gives his view of the 
difficult subject of a true thermometrie scale; which 
he does not suppose to be correctly represented by 
the simple expansion of any known substance; but 
that the gases expand with true increments of heat 
in a very slow geometrical progression, whilst li- 
quids expand as the squares of the true temperatures 
from their freezing points. These and other laws 
equally arbitrary have not received support from 
later and more precise experiments; and in the se- 
cond volume of his work (1827) he freely acknow- 
ledges the correction of his hypothesis by the more 
recent French experiments, 
II. I now proceed to the other part of Dalton’s 
Mr Gra- 
ham’s law 
of diffu- 
sion, 
(617.) 
Dalton on 
the expan- 
sion of the 
gases by 
heat. 
(618.) 
Dalton’s 
labours on which his reputation is principally based ; 4 somic 
namely, the clear assertion and experimental esta- Theory. 
blishment of the Atomic Theory or doctrine of chemi- 
cal equivalents. I introduce it here on account of 
its important bearing on all physical questions in 
which the constitution of matter and the forces act- 
ing at minute distances are involved. 
Early opinions on the constitution of matter and 
(619.) 
chemical combination.—Two opinions have prevailed Early opi- 
from the very earliest times respecting the constitu- 
tion of body ;—that which su 
nions on 
the consti- 
oses its entire homo- tution of 
geneousness, and that which allows that it consists matter. 
of material parts or atoms separated by void spaces ; 
these parts or atoms being indivisible. This last is 
the doctrine of Democritus and Epicurus, and in 
modern times of Bacon, Newton, and Dalton.1 The 
former opinion has been held by Leibnitz, and many 
2 The reader will find ample details on the opinions of the older philosophers in Daubeny’s Introduction to the Atomic Theory, 
