[Thomson gives a table of specifc gravities of a number of different gases at constant temperature and pressure , which differ widely. He then continues:]
6. 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 weight of the atoms, differs in different gases. The first supposition is by no means probable, supposing the size and weight 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 weight of the atoms of different gases to differ, this in reality includes the first cause likewise; for every variation in size and weight 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 weight 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 weight 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 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 a particle of water is formed by the combination of one atom of oxygen with one atom of hydrogen. If we represent an atom of oxygen, hydrogen, and azote, by the following symbols,
| Oxygen | O |
| Hydrogen | ⊙ |
| Azote | ⊕ |
Then a particle of water will be represented by the following symbol:
| Water ⊙O |
But if this hypothesis be allowed, it furnishes us with a ready method of ascertaining the relative weight of those atoms that enter into such combinations; for it
* Mr Dalton has not yet published a detailed account of his hypothesis, though he has noticed it in the 1st Volume of his New System of Chemical Philosophy, published in 1808. The full explanation is reserved for the second volume of that Work, which has not yet made its appearance.
has been proved by analysis, that water is composed of 85 2/3 of oxygen and 14 1/3 of hydrogen. A particle of parts by weight of water of course is composed of 85 2/3 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. Thus we have obtained the following relative weights of these two elementary bodies.
| Hydrogen | 1 |
| Oxygen | 6 |
To find out the weight of an atom of azote, we may examine its combinations with oxygen. 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 nitric acid con- sists of nitrous gas and oxygen, united atom to atom; then the following will be the symbols and constituents of these three bodies:
| Nitrous gas | O⊕ |
| Nitrous oxide | ⊕O⊕ |
| Nitric acid | O⊕O |
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 weight of their elements as deduced from the hypothesis.
Nitrous gas is composed of 1.00 azote and 1.36 oxygen, or of 6 oxygen and 4.41 azote. This would make the weight of an atom of azote about 4 1/2.
Nitrous oxide is composed of 2 azote and 1.174 oxygen, or of 6 oxygen and 5.11 + 5.11 azote.
Nitric acid of 1 azote and 2.36 oxygen, or of 6+6 oxygen and 5.08 azote.
These three give us the relative weights of oxygen and
azote as follows, reckoning the weight of oxygen 6 as before:
| Oxygen. | Azote. |
| 6 : | 4.41 |
| : | 5.11 |
| : | 5.08 |
The weight of an atom of azote, as deduced from the analysis of these three bodies, does not come out exactly the same for each. Yet the difference is not great, and surely not greater than might have been expected from the difficulty of making such minute experiments with precision. The mean of the whole gives the weight of an atom of azote 4.87. We may therefore take 5 as a convenient whole number without deviating far from the truth.
On the supposition that the hypothesis of Mr Dalton is well founded, the following Table exhibits the weight 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 |
| O | Oxygen | 6 |
| ⊗ | Muriatic acid | 18 |
| ⊙O | Water | 7 |
| O⊕ | Nitrous gas | 11 |
| ⊕O⊕ | Nitrous oxide | 16 |
| O⊕O | Nitric acid | 17 |
[This table is found in the 1809 french translation of the 3rd (1807) edition, vol. 5, p. 298, containing in addition oxymuriatic acid and superoxymuriatic acid.]
The weight of the remaining gases, into the composition of which the atoms of solid bodies enter, will come under our consideration in a subsequent Section.
I have purposely omitted the consideration of am-
monia, because its composition has not been ascertained with sufficient precision to admit of calculation. It was considered as composed of 80 parts azote and 20 hydrogen, and of course as binary compound of one atom of azote and one atom of hydrogen. But the late experiments of Davy and Berzelius have shown that oxygen is also one of its constituents. But the proportion of this last ingredient is so small, that in order to make the weight of the constituents of ammonia tally with the numbers deduced above, it would be necessary to consider it as a compound of a considerable number of atoms. The subject is still very obscure, and not yet ripe for investigation. It may probably lead to views that will overturn a great part of our presently received Chemical Philosophy. But as it does not materially affect the hypothesis of Mr Dalton, it is not necessary to enter upon the subject here.