In the year 1802 Dalton conducted an examination of air from various localities, and concluded that one hundred volumes of air are composed of twenty-one volumes of oxygen and seventy-nine volumes of nitrogen. This appears to have been his first piece of purely chemical work. But in the next year he again returns to physical phenomena. In the paper already referred to, on the absorption of gases by water and other liquids, published in this year, he had stated that "All gases that enter into water and other liquids by means of pressure, and are wholly disengaged again by the removal of that pressure, are mechanically mixed with the liquid, and not chemically combined with it." But if this be so, why, he asked, does not water mechanically dissolve the same bulk of every kind of gas? The answer which he gives to this question is found at the close of the paper; to the student of chemistry it is very important:—
"This question I have duly considered, and though I am not yet able to satisfy myself completely, I am nearly persuaded that the circumstance depends upon the weight and number of the ultimate particles of the several gases, those whose particles are lightest and single being least absorbable, and the others more, accordingly as they increase in weight and complexity. An inquiry into the relative weights of the ultimate particles of bodies is a subject, as far as I know, entirely new. I have lately been prosecuting this inquiry with remarkable success. The principle cannot be entered upon in this paper; but I shall just subjoin the results, as far as they appear to be ascertained by my experiments." Then follows a "Table of the relative weights of the ultimate particles of gaseous and other bodies." The following numbers, among others, are given:—
| Hydrogen | 1 | Sulphur | 14·4 | |
| Oxygen | 5·5 | Alcohol | 15·1 | |
| Azote | 4·2 | Nitrous oxide | 13·7 | |
| Phosphorus | 7·2 | Ether | 9·6 |
Here is the beginning of the atomic theory; and yet Dalton's strictly chemical experimental work lies in the future. The scope of the theory is defined in that sentence—"An inquiry into the relative weights of the ultimate particles of bodies." His paper on mixed gases is illustrated by a plate,[7] which shows how vividly Dalton at this time pictured to himself a quantity of gas as composed of many little particles, and how clearly he recognized the necessity of regarding all the particles of each elementary gas as alike, but as differing from those of every other elementary gas.
In 1804 Dalton was invited to deliver a course of lectures in the Royal Institution of London, on heat, mixed gases and similar subjects. In these lectures he expounded his views on the constitution of gases, on absorption of gases by liquids, etc. These views drew much attention in this and other countries. "They are busy with them," he writes in 1804, "at London, Edinburgh, Paris and in various parts of Germany, some maintaining one side and some another. The truth will surely out at last."
Fig. 2
Dalton's love of numerical calculations is noticeable in a trivial circumstance which he mentions in a letter from London to his brother. He tried to count the number of coaches which he met in going to the Friends' morning meeting: this he assures his brother he "effected with tolerable precision. The number was one hundred and four."
During vacation time Dalton usually made a walking excursion in the Lake district. He was extremely fond of mountain scenery, but generally combined the pursuit of science with that of pleasure; he carried his meteorological instruments with him, determined the dew-point at various altitudes, and measured mountain heights by the aid of his barometer. Sometimes however he refused to have anything to do with science. A companion in one of these excursions says that he was "like a schoolboy enjoying a holiday, mocking the cuckoos, putting up and chasing the hares, stopping from time to time to point out some beautiful view, or loitering to chat with passing pedestrians."
This side of Dalton's nature was not often apparent. In him the quiet, hard-working student generally appeared prominently marked; but on the half-holiday which he allowed himself on each Thursday afternoon, in order to enjoy the society of a few friends and to engage in his favourite amusement of a game at bowls, he laid aside something of the quietness, regularity and decorum which usually characterized him. "When it came to his turn to bowl he threw his whole soul into the game,... and it was not a little amusing to spectators to see him running after the ball across the green, stooping down as if talking to it, and waving his hands from one side to the other exactly as he wished the line of the ball to be, and manifesting the most intense interest in its coming near to the point at which he aimed."