Dalton's work during the next six or eight years dealt chiefly with problems suggested by his meteorological observations; he published a volume on "Meteorological Observations and Essays," chiefly occupied with descriptions of the instruments employed, more especially of the thermometer and barometer, and an instrument for determining the dew-point of air. By this time he had established the existence of a connection of some kind between magnetism and the aurora, and had thus laid the foundations of a most important branch of meteorology.
In 1799, in a note to a paper on rain and dew, he begins his work on aqueous vapour in the atmosphere by proving that water vapour exists as such in the air. This paper is quickly followed by another on the conducting power of water for heat.
A very important paper was published in 1801, on the "Constitution of Mixed Gases, etc.," wherein Dalton asserted that the total pressure of a mixture of two gases on the walls of the containing vessel is equal to the sum of the pressures of each gas; in other words, that if one gas is removed the pressure now exerted by the remaining gas is exactly the same as was exerted by that gas in the original mixture. In a paper published much later (1826), when his views and experiments on this subject were matured, he writes: "It appears to me as completely demonstrated as any physical principle, that whenever two or more ... gases or vapours ... are put together, either into a limited or unlimited space, they will finally be arranged each as if it occupied the whole space, and the others were not present; the nature of the fluids and gravitation being the only efficacious agents."
This conclusion was followed out and extended in a paper published in 1803, on the absorption of gases by water and other liquids, wherein he states that the amount of each gas _mechanically dissolved_ by a liquid from a mixture of gases depends only on the quantity of _that_ gas in the mixture, the other gases exerting no influence in this respect.
Dalton now considered the variation in the pressures of various gases caused by increasing or decreasing temperature, and then proceeded to discuss the relations which exist between the volumes of gases and the temperature at which these volumes are measured. He concluded that "all elastic fluids" under the same pressure expand equally by heat: and he adds the very important remark, "It seems, therefore, that general laws respecting the absolute quantity and the nature of heat are more likely to be derived from the study of elastic fluids than of other substances"--a remark the profound truth of which has been emphasized by each step in the advances made in our conception of the nature of heat since the time of Dalton.