Van Helmont (1577 or 80 to 1644), of Brussels, was distinctly less mystical and alchemical than Paracelsus a century earlier. Already he illustrates the preoccupation with heat and with gases. It was his great service to concentrate chemical attention on these latter, and to realise definitely that there are several kinds ; though he could not usually give clear tests or definitions for them. He also knew the three chief mineral acids, which we call sulphuric, hydrochloric, and nitric, and knew that metals dissolved in them can be recovered.

Fludd in 1617 described enclosed combustion in an inverted glass vessel over water—a valuable device for manipulation, which showed that after a certain volume-diminution has occurred, the flames go out.

Van Helmont knew of this latter fact. He distinguished gases from the more easily liquefied vapours. He denied that fire is a separate " element "—merely gas at a high temperature. He is less modern in professing to have effected transmutation of metals and in particularising the "archaei "which in his view presided over the various bodily systems, nervous, digestive, respiratory, etc., and in arranging them in a hierarchy. Yet even here his ideas had a new definiteness which ensured their ultimate criticism. Thus, he tried to identify digestion with a definite if uncomprehended process of transformation, fermentation, on which he had modern- sounding views. He assimilated other transformation processes of this sort with the type-case of the leavening of bread.

His quantitative use of the balance—his stress on the importance of weight-changes—was long in getting the imitation it deserved. It did not become a habit of chemists until well into the 18th century.

Yet he was still concerned with the old alchemical elements, and with transmutation. With Thales, and against Paracelsus, he reduced the four to two : air and water, with stress on the latter. One of his experiments in support of this well illustrates the enterprise, yet the quaintness, of the science of the time. For five years he watered a weighed willow in a weighed quantity of earth. He compared weights before and after. The weight of the earth was unchanged. From this he concluded, not unreasonably, that all the willow's increase was due to the (all- important) " element " water. Clearly, the full part played by gases was still unrealised when his work was posthumously published in 1648 !