Early study of the nature of the air. Before the time of Aristotle, a famous Greek who lived in the fourth century BC, it was generally believed that air was composed of just one substance. Aristotle argued that air was not merely one substance but a mixture of two. One of these substances he thought was moisture.
The other he could not name, though he felt sure that it existed. Unlike the earlier belief that the air was a simple substance, Aristotle’s conclusion was in part scientific because he based it on his observations of clouds and rain. He knew that these were formed from the moisture in the atmosphere.
He concluded, therefore, that moisture must be part of the air. Exact knowledge of the true nature of air began with the work of Priestley, an English scientist, in the seventeenth century.