A System of Chemistry. By Thomas Thomson, M.D., F.R.S.E., 3rd edition, vol. iii., Edinburgh 1807, pp. 424-429 and 451-452.

Copied from Alembic Club Reprint #2

These extracts form a part of the first public exposition of Dalton's views. The pagination is from the Alembic Club's reprint.

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This difference between the density of the gases, while their elasticity is the same, must be owing to one of two causes: Either the repulsive force or the density of the atoms, differs in different gases. The first supposition is by no means probable, supposing the size and density of the particles of different gases the same, and indeed would but ill agree with the analogy of nature; but the second is very likely to be the true cause. And if we suppose the size and density of the atoms of different gases to differ, this in reality includes the first cause likewise; for every variation in size and density must necessarily occasion a corresponding variation in the repulsive force, even supposing that force abstractedly considered to be the same in all.

We have no direct means of ascertaining the density of the atoms of bodies ; but Mr Dalton, to whose uncommon ingenuity and sagacity the philosophic world is no stranger, has lately contrived an hypothesis which, if it prove correct, will furnish us with a very simple method of ascertaining that density with great precision, Though the author has not yet thought fit to publish his hypothesis, yet as the notions of which it consists are original and extremely interesting, and as they are intimately connected with some of the most intricate parts of the doctrine of affinity, I have ventured, with Mr

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Dalton's permission, to enrich this work with a short sketch of it. *

The hypothesis upon which the whole of Mr Dalton's notions respecting chemical elements is founded, is this: When two elements unite to form a third substance, it is to be presumed that one atom of one joins to one atom of the other, unless when some reason can be assigned for supposing the contrary. Thus oxygen and hydrogen unite together and form water. We are to presume that an atom of water is formed by the combination of one atom of oxygen with one atom of hydrogen. In like manner one atom of ammonia is formed by the combination of one atom of azote with one atom of hydrogen. If we represent an atom of oxygen, hydrogen, and azote, by the following symbols,

Oxygen . . . . . . .
Hydrogen . . . . .
Azote . . . . . . . .

Then an atom of water and of ammonia will be represented respectively by the following symbols:

Water . . . . . . .
Ammonia . . . .

But if this hypothesis be allowed, it furnishes us with a ready method of ascertaining the relative density of those atoms that enter into such combinations; for it has been proved by analysis, that water is composed of

* In justice to Mr Dalton, I must warn the reader not to decide upon the notions of that philosopher from the sketch which I have given, derived from a few minutes' conversation, and from a short written memorandum. The mistakes, if any occur, are to be laid to my account, and not to his ; as it is extremely probable that I may have misconceived his meaning in some points.

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82 2/3 of oxygen and 14 1/3 of hydrogen. An atom of water of course is composed of 85 2/3 parts by weight of oxygen and 14 1/3 parts of hydrogen. Now, if it consist of one atom of oxygen united to one atom of hydrogen, it follows, that the weight of one atom of hydrogen is to that of one atom of oxygen as 14 1/3 to 85 2/3, or as 1 to 6 very nearly. In like manner an atom of ammonia has been shown to consist of 80 parts of azote and 20 of hydrogen. Hence an atom of hydrogen is to an atom of azote as 20 to 80, or as 1 to 4. Thus we have obtained the following relative densities of these three elementary bodies.

Hydrogen . . . . . 1
Azote . . . . . . . . 4
Oxygen . . . . . . 6

We have it in our power to try how far this hypothesis is consonant to experiment, by examining the combination of azote and oxygen, on the supposition that the bodies unite, atom to atom, and that the respective densities of the atoms are as in the preceding table. But azote and oxygen unite in various proportions, forming nitrous oxide, nitrous gas, and nitric acid, besides some other compounds which need not be enumerated. The preceding hypothesis will not apply to all these compounds ; Mr Dalton, therefore, extends it farther. Whenever more than one compound is formed by the combination of two elements, then the next simple combination must, he supposes, arise from the union of one atom of the one with two atoms of the other. If we suppose nitrous gas, for example, to be composed of one atom of azote, and one of oxygen, we shall have two new compounds, by uniting an atom of nitrous gas to an atom of azote, and to an atom of oxygen, respectively. If we suppose farther, that nitrous oxide is composed of an atom of nitrous gas and an atom of azote, while

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nitric acid consists of nitrous gas and oxygen, united atom to atom; then the following will be the symbols and constituents of these three bodies:

Nitrous gas . . . . . . .
Nitrous oxide . . . . .
Nitric acid . . . . . . . .

The first gas consists only of two atoms, or is a binary compound, but the two others consist of three atoms, or are ternary compounds ; nitrous oxide contains two atoms of azote united to one of oxygen, while nitric acid consists of two atoms of oxygen united to one of azote.

When the atoms of two elastic fluids join together to form one atom of a new elastic fluid, the density of this new compound is always greater than the mean. Thus the density of nitrous gas, by calculation, ought only to be 1.045 but its real density is 1.094. Now as both nitrous oxide and nitric acid are specifically heavier than nitrous gas, though the one contains more of the lighter ingredient, and the other more of the heavier ingredient than that compound does, it is reasonable to conclude, that they are combinations of nitrous gas with azote and oxygen respectively, and that this is the reason of the increased specific gravity of each ; whereas were not this the case, nitrous oxide ought to be specifically lighter than nitrous gas. Supposing, then, the constituents of these gases to be as represented in the preceding table, let us see how far this analysis will correspond with the densities of their elements, as above deduced from the compositions of water and ammonia.

Nitrous gas is composed of 1.00 azote and 1.36 oxygen, or of 4 azote and 5.4 oxygen.

Nitrous oxide, of 2 azote and 1.174 oxygen, or of 4 + 4 azote and 4.7 oxygen.

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Nitric acid, of 1 azote and 2.36 oxygen, or of 4 azote and 4.7 + 4.7 oxygen. These three give us the following as the relative densities of azote and oxygen:

Azote. Oxygen.
4 : 5.4
  : 4.7
  : 4.7

The mean of the whole is nearly 4 : 5 ; but from preceding analyses of water and ammonia, we obtained their densities 4 : 6. Though these results do not correspond exactly, yet the difference is certainly not very great, and indeed as little as can reasonably be expected, even supposing the hypothesis is well founded, if we consider the extreme difficulty of attaining accuracy in the analysis of gaseous compounds. If ammonia were supposed a compound of 83 azote and 17 hydrogen, instead of 80 azote and 20 hydrogen, in that case the density of azote would be five instead of four, and the different sets of experiments would coincide very nearly. Now it is needless to observe how easy it is, in analysing gaseous compounds, to commit an error of 3 per cent. which is all that would be necessary to make the different numbers tally.

On the supposition that the hypothesis of Mr Dalton is well founded, the following table exhibits the density of the atoms of the simple gases, and of those which are composed of elastic fluids, together with the symbols of the composition of these compound atoms:

Hydrogen . . . . . . . .1
Azote . . . . . . . . . . .5
Oxygen . . . . . . . . . .6
Muriatic acid . . . . . . .9

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Water . . . . . . . . . . . . . . 7
Ammonia . . . . . . . . . . . 6
Nitrous gas . . . . . . . . . .11
Nitrous oxide . . . . . . . . 16
Nitric acid . . . . . . . . . . 17
Oxymuriatic acid . . . . .24
Hyperoxymuriatic acid . .27

Oxygen and nitrous gas, when brought into contact, immediately unite and form a yellow-coloured vapour. From the experiments of Dalton, it appears that these gases are capable of uniting in two different proportions. One hundred measures of common air, when added to 36 measures of nitrous gas in a narrow tube over water leave a residue of 79 inches; and one hundred measures of common air, admitted to 72 measures of nitrous gas in a wide glass vessel over water, leave likewise a residue of 79 measures.* According to these experiments, 21 cubic inches of oxygen gas is capable of uniting with 36 and with 72 cubic inches of nitrous gas; or 100 inches of oxygen unite with 171.4 and with 342.8 inches of nitrous gas. If we apply Mr Dalton's hypothesis, stated in a former Section, to these combinations, we shall have the first composed of one atom of oxygen united to one atom of nitrous gas; the second, of one atom of oxygen united to two atoms of nitrous gas. The first appears to be the substance usually distinguished by the name of nitric acid ; the second is nitrous vapour, or nitric acid saturated with nitrous gas. The following will be the

* Phil. Mag. xxiii. 351.

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symbols denoting the composition of an atom of each, and the density of that atom, obtained by adding together the numbers denoting the density of each of the constituent atoms.

Nitric acid . . . . . . . .17
Nitrous vapour . . . . 28

The first is a triple compound, and can only be resolved into nitrous gas and oxygen, or into azote and oxygen - but the second is a quintuple compound, and may be resolved into nitrous oxide and oxygen, nitrous gas and oxygen, nitric acid and nitrous gas, oxygen and azote.