Until about the end of the sixteenth century chemists, or alchemists more properly, still held to the theory of Aristotle that the four primary properties of matter were fire, air, water, and earth.
At that period, however, some such experimenters as Galileo, Harvey, and others were beginning to discard the old theories, and the alchemists themselves were finding flaws in their ancient beliefs.
Among the critics was the Belgian physician, Jean Baptiste van Helmont, born in 1577. Although he believed that he had himself transmuted mercury into gold with a small piece of the “philosopher’s stone,” he was obliged to fall back upon supernatural agencies to find an explanation for certain phenomena that he discovered in his other experiments.
He observed that when he applied heat to certain things, water especially, a vapor would arise. He believed this vapor to be fundamentally water in ultrararefied form, and, as he says, “for want of a name, I have called that vapor gas, not far disassociated from the chaos of the ancients.”
Thus, although van Helmont had no more than a vague understanding of the nature of gases, we are indebted to him for the word.