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." [Illustration: 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."