Atmospheric Water Molecules and the Morning Dew

A decade before he enunciated his atomic theory, Dalton had found that the amount (pressure) of water vapor in air or introduced into a vacuum depended solely on the temperature of the liquid water in equilibrium. (Dalton also developed the concept of the dewpoint). This suggested that water vapor did not form a chemical compound with air (else, why would it enter an evacuated vessel ). It also suggested that water s vapor pressure and very existence were completely independent of other gases in the air. In 1801, his experimental studies permitted a more general statement of what we now call Dalton s law of partial pressures  

When two elastic fluids, denoted by A and B, are mixed together, there is no mutual repulsion amongst their particles that is, the particles of A do not repel those of B, as they do one another. Consequently, the pressure or whole weight upon any one particle arises solely from those of its own kind. 

This is an interesting, if not very straightforward notion. Why should particles of A (e.g. nitrogen) repel other A particles but not particles of B, such as oxygen, toward which they remain indifferent  

Dalton s explanation mixes Lavoisier s caloric theory with Newton s mechanical repulsion theory and then adds a dash of his own special ingredient. First, it is important to recall that when combined in a metal oxide or calx, the element oxygen is in its fixed state. Thus, according to Lavoisier, oxygen gas is actually oxygenated caloric since heat was required in order to free the ele- 

Not only is air uniformly mixed at sea level, rather than consisting completely or mostly of the denser gas, oxygen, but the same mixture persists high into the atmosphere. This was known at the time Dalton was formulating his atomic theory. In 1804, Joseph Louis Gay-Lussac piloted a balloon some 23,000 

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