Found in a collection of Papers from the History of Chemisty, put together by Carmen Giunta (of Le Moyne College)
John Dalton (
1766-
1844)
A New System of Chemical
Philosophy
excerpts, (Manchester, 1808) {from facsimile edition (London: Dawson)}
Chap. I. On
Heat or Caloric
The most probable opinion concerning the nature of caloric, is, that of its being an elastic fluid of great
subtility, the particles of which repel one another, but are attracted by all other bodies.
When all surrounding bodies are of one temperature, then the heat attached to them is in a quiescent state; the
absolute quantities of heat in any two bodies in this case are not equal, whether we take the bodies of equal
weights or of equal bulks. Each kind of matter has its peculiar affinity for heat, by which it requires a certain
portion of the fluid, in order to be in equilibrium with other bodies at a certain temperature. Were the whole
quantities of heat in bodies of equal weight or bulk, or even the relative quantities, accurately ascertained, for
any temperature, the numbers expressing those quantities would constitute a table of specific heats, analogous
to a table of specific gravities, and would be an important acquisition to science. Attempts of this kind have
been made with very considerable success.
Whether the specific heats, could they be thus obtained for one temperature, would express the relation at
every other temperature, whilst the bodies retained their form, is an enquiry of some moment. From the
experiments hitherto made there seems little doubt of its being nearly so; but it is perhaps more correct to
deduce the specific heat of bodies from equal bulks than from equal weights. It is very certain that the two
methods will not give precisely the same results, because the expansions of different bodies by equal
increments of temperature are not the same. But before this subject can well be considered, we should first
settle what is intended to be meant by the word temperature.
...
SECTION 4. THEORY OF THE SPECIFIC HEAT OF ELASTIC FLUIDS.
Since the preceding section was printed off, I have spent some time in considering the constitution of elastic
fluids with regard to heat. The results already obtained cannot be relied upon; yet it is difficult to conceive and
execute experiments less exceptionable than those of Crawford. It is extremely important, however, to obtain
the exact specific heat of elastic fluids, because the phenomena of combustion and of heat in general, and
consequently a great part of chemical agency, are intimately connected therewith.
In speaking of the uncertainty of Crawford's results on the specific heat of elastic fluids, it must not be
understood that all of them are equally implicated. The reiterated experiments on the heat given out by the
combustion of hydrogen, in which it was found that 11 measures of mixed gases, when fired by electricity
heated 20.5 measures of water 2°.4 (page 263) at a medium, were susceptible of very considerable accuracy, and
are therefore entitled to credit. The comparative heat of atmospheric air and water, which rested on the
observance of nearly 1/4 of a degree of temperature, is probably not very far from the truth; but the very small
difference in the heats communicated by equal bulks of oxygen, hydrogen, carbonic acid, azotic gas and
common air, together with the great importance of those differences in the calculation, render the results very
uncertain. He justly observes, that if we suppose the heats imparted by equal bulks of these gases to be equal, it
will not affect his doctrine. The tenor of it necessarily led him to estimate the heat of oxygen high, compared
with equal weights of carbonic acid and aqueous vapour, and of azotic gas or phlogisticated air, as it was then
called, under the idea of its being an opposite to oxygen or dephlogisticated air. Indeed his deductions
respecting azotic gas, are not consistent with his experiments: for he makes no use of experiments 12 and 13,
which are the only direct ones for the purpose, but he infers the heat of azotic gas from the observed difference
between oxygen and common air. The result gives it less than half that of common air; whereas from the 13th
experiment, scarcely any sensible difference was perceived between them. He has in all probability much
underrated it; but his errors in this respect whatever they may be, do not affect his system.
When we consider that all elastic fluids are equally expanded by temperature, and that liquids and solids are not
so, it should seem that a general law for the affection of elastic fluids for heat, ought to be more easily deducible
and more simple than one for liquids, or solids. --There are three suppositions in regard to elastic fluids which
merit discussion.
1. Equal weights of elastic fluids may have the same quantity of heat under like circumstances of temperature
and pressure.
The truth of this supposition is disproved by several facts: oxygen and hydrogen upon their union give out
much heat, though they form steam, on elastic fluid of the same weight as the elements composing it. Nitrous
gas and oxygen unite under similar circumstances. Carbonic acid is formed by the union of charcoal, a
substance of low specific heat, with oxygen; much heat is given out, which must be principally derived from the
oxygen; if then the charcoal contain little heat, and the oxygen combining with it be reduced, the carbonic acid
must be far inferior in heat to an equal weight of oxygenous gas.
2. Equal bulks of elastic fluids may have the same quantity of heat with the same pressure and temperature.
This appears much more plausible; the diminution of volume when a mixture of oxygen and hydrogen is
converted into steam, may be occasioned by a proportionate diminution of the absolute heat; the same may be
said of a mixture of nitrous gas and oxygen. The minute differences observed by Crawford, may have been
inaccuracies occasioned by the complexity of his experiments. --But there are other considerations which
render this supposition extremely improbable, if they do not altogether disprove it. Carbonic acid contains its
own bulk of oxygen; the heat given out at its formation must therefore be exactly equal to the whole heat
previously contained in the charcoal on this supposition; but the heat by the combustion of one pound of
charcoal seems, at least, equal to the heat by the combustion of a quantity of hydrogen sufficient to produce one
pound of water, and this last is equal to, or more than the heat retained by the water, because steam is nearly
twice the density of the elastic mixture from which it is produced; it should therefore follow, that charcoal
should be found of the same specific heat as water, whereas it is only about 1/4 of it. Were this supposition true,
the specific heats of elastic fluids of equal weights would be inversely as the specific gravities. --If that of
steam or aqueous vapour were represented by 1, oxygen would be .64, hydrogen 8.4, azote .72, and carbonic acid
.46. --But the supposition is untenable.
3. The quantity of heat belonging to the ultimate particles of all elastic fluids, must be the same under the same
pressure and temperature.
It is evident the number of ultimate particles of molecules in a given weight or volume of one gas is not the
same as in another: for, if equal measures of azotic and oxygenous gases were mixed, and could be instantly
united chemically, they would form nearly two measures of nitrous gas, having the same weight as the two
original measures; but the number of ultimate particles could at most be one half of that before the union. No
two elastic fluids, probably, therefore, have the same number of particles, either in the same volume or the
same weight. Suppose, then, a given volume of any elastic fluid to be constituted of particles, each surrounded
with an atmosphere of heat repelling each other through the medium of those atmospheres, and in a state of
equilibrium under the pressure of a constant force, such as the earth's atmosphere, also at the temperature of
the surrounding bodies; suppose further, that by some sudden change each malecule (sic) of air was endued
with a stronger affinity for heat; query the change that would take place in consequence of this last
supposition? The only answer that can be given, as it appears to me, is this. --The particles will condense their
respective atmospheres of heat, by which their mutual repulsion will be diminished, and the external pressure
will therefore effect a proportionate condensation in the volume of air: neither an increase nor diminution in
the quantity of heat around each malecule, or around the whole, will take place. Hence the truth of the
supposition, or as it may now be called, proposition, is demonstrated.
Corol. 1. The specific heats of equal weights of any two elastic fluids, are inversely as the weights of the atoms
or molecules.
2. The specific heats of equal bulks of elastic fluids, are directly as their specific gravities, and inversely as the
weights of their atoms.
3. Those elastic fluids that have their atoms the most condensed, have the strongest attraction for heat; the
greater attraction is spent in accumulating more heat in a given space or volume, but does not increase the
quantity around any single atom.
4. When two elastic atoms unite by chemical affinity to form one elastic atom, one half of their heat is
disengaged. When three unite, then two thirds of their heat is disengaged, &c. And in general, when m elastic
particles by chemical union become n; the heat given out is to the heat retained as m-n is to n.
One objection to this proposition it may be proper to obviate: it will be said, an increase in the specific
attraction of each atom must produce the same effect on the system as an increase of external pressure. Now
this last is known to express or give out a quantity of the absolute heat; therefore the former must do the same.
This conclusion must be admitted; and it tends to establish the truth of the preceding proposition. The heat
expressed by doubling the density of any elastic fluid amounts to about 50°, according to my former
experiments; this heat is not so much as one hundredth part of the whole, as will be shewn hereafter, and
therefore does not materially affect the specific heat: it seems to be merely the interstitial heat amongst the
small globular molecules of air, and scarcely can be said to belong to them, because it is equally found in a
vacuum of space devoid of air, as is proved by the increase of temperature upon admitting air into a vacuum.
Before we can apply this doctrine to find the specific heat of elastic fluids, we must first ascertain the relative
weights of their ultimate particles. Assuming at present what will be proved hereafter, that if the weight of an
atom of hydrogen be 1, that of oxygen will be 7, azote 5, nitrous gas 12, nitrous oxide 17, carbonic acid 19,
ammoniacal gas 6, carburetted hydrogen 7, olefiant gas 6, nitric acid 19, carbonic oxide 12, sulphuretted
hydrogen 16, muriatic acid 22, aqueous vapour 8, ethereal vapour 11, and alcoholic vapour 16; we shall have the
specific heats of the several elastic fluids as in the following table. In order to compare them with that of water,
we shall further assume the specific heat of water to that of steam as 6 to 7, or as 1 to 1.166.
Table of the specific heats of elastic fluids.
Hydrogen 9.382
Olefiant gas 1.555
Azote 1.866
Nitric acid .491
Oxygen 1.333
Carbonic oxide .777
Atmos. air 1.759
Sulph. hydrogen .583
Nitrous gas .777
Muriatic acid .424
Nitrous oxide .549
Aqueous vapour 1.166
Carbonic acid .491
Ether. vapour .848
Ammon. gas 1.555
Alcohol. vapour .586
Carb. hydrogen 1.333
Water 1.000
Let us now see how far these results will accord with experience. It is remarkable that the heat of common air
comes out nearly the same as Crawford found it by experiment; also, hydrogen excels all the rest as he
determined; but oxygen is much lower and azote higher. The principles of Crawford's doctrine of animal heat
and combustion, however, are not at all affected with the change. Besides the reason for thinking that azote has
been rated too low, we see from the Table, page 62, that ammonia, a compound of hydrogen and azote, has a
higher specific heat than water, a similar compound of hydrogen and oxygen.
Upon the whole, there is not any established fact in regard to the specific heats of bodies, whether elastic or
liquid, that is repugnant to the above table as far as I know; and it is to be hoped, that some principle analogous
to the one here adopted, may soon be extended to solid and liquid bodies in general