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CHEMISTRY 359 system. This was inaugurated by Paracelsus, in the first quarter of the 16th century. It fills a well defined period in chemical history, en- during till the middle of the 17th century. The views regarding the transmutation of met- als remained unchanged, but gold-making was no longer the chief aim of the leading chemists. Indeed alchemy, as an art, soon became sepa- rated from chemistry. The characteristic of this period was the intimate connection be- tween medicine and chemistry, the whole pur- pose of the latter being to cure disease. As its purpose was thus elevated, it passed into the hands of educated men, through whom the amount of chemical knowledge was rapidly in- creased. Paracelsus regarded sulphur, mercu- ry, and salt as the elements of all substances, organic and inorganic. His ideas of sulphur and mercury correspond with those of the earlier alchemists, while that of salt is opposed to that of mercury, the former being typical of solid- ity and incombustibility, while the latter is expressive of volatility without decomposition. Besides giving a great impulse to the science, Paracelsus introduced numberless chemical preparations into use in medicine, rendering them familiar to physicians and apothecaries, whence the latter were often induced to occu- py themselves with the study of chemistry. The controversies between the followers of Paracelsus and his adversaries also excited great interest in the science. At this epoch Agricola (1490-1555) flourished in Saxony. Standing aloof from the great chemical ques- tions of the day, he occupied himself almost ex- clusively with metallurgy, in which specialty he became very distinguished by his writings. He first describes clear processes of smelting metals and of assaying ores. He may also be regarded as the founder of this branch of chemical science. Distinguished among the opposers of the mystical style of the alchemists, and for his exposition of their charlatanry, was Libavius (died 161 6). Many valuable observa- tions are also due to him. He prepared sul- phuric acid by burning sulphur with salpetre, and proved its identity with the acid obtained by roasting green vitriol or alum. By distil- ling tin with corrosive sublimate he prepared bichloride of tin, still known as the fuming liquor of Libavius. The ideas concerning ele- ments entertained by Van Helmont (1577-1644) differ essentially from those of preceding chem- ists. He rejected the four elements of Aris- totle, for fire is not a substance ; heat and cold are only abstract qualities, and not material things; therefore fire cannot be contained in any substance as a material component. He denounced, on the other hand, the elements of the alchemists, sulphur, mercury, and salt, and especially the theory that they were elements of the animal body, in proof of which, according to him, no facts existed. He regarded water as the chief ingredient of all things. It could be obtained by burning any combustible body. From it all parts of vegetables are formed, their earthy as well as combustible portions. In proof of this, Van Helmont made the following experiment : A willow twig weighing 5 Ibs. was planted in a pot containing a known quantity of thoroughly dried earth ; this was covered to protect it from dust, and watered daily with rain water during five years. The willow, mean- while, grew large and strong, weighing 164 Ibs., although the earth in the pot, when again dried, had only lost 2 oz. This was regarded as proof positive that water alone forms plants and the mineral matters contained in them, while ani- mals obtain their components directly from plants. Van Helmont introduced the term gas into chemistry, described several different kinds of gases, and distinguished them from vapors. As the practical chemist of this period, Glauber (1604-'68) stands preeminent. By improving the processes of preparing the mineral acids, by the discovery of salts, and by observing many new facts, he did great service. By synthesis he obtained a knowledge of the composition of many substances. It is worthy of remark that, during the latter half of the 17th century, Tache- nius made the first approximately correct quan- titative chemical statement which occurs in the history of the science, viz. : that metallic lead when burned to red lead increases its weight by y 1 ^. In the latter half of the 17th century the foundation of several learned societies pro- moted and advanced the study of chemistry as well as of the other sciences. During the continuance of this period of intimate con- nection between chemistry and medicine a great amount of chemical knowledge had been collect- ed ; the metals and their compounds were well known, as were the three principal mineral acids and their combinations with the alkalies. Toward its close a mass of observations, the material with which the structure of modern chemistry has been reared, invited explanation and classification. Important views were at this period advanced by Boyle (1626-'91). He first treated the question of elements from the same point of view as has been taken by mod- ern chemists. He proved more clearly than any of his predecessors that the four elements of Aristotle were inadmissible, and how little the elements of the alchemists were calculated to afford a rational conception of the composi- tion of bodies. He thought that, rather than seek to explain the primary elements of matter, which admit of various views, attention ought to be specially directed to those ingredients which can be separated and isolated as such. If these cannot be further decomposed, they should be called elements, although he admits that they may be still further decomposed as knowledge increases. This clear definition was in his eyes much better calculated to advance the progress of chemical knowledge than the vague ideas expressed in the theories of Aris- totle and the alchemists. Boyle's views, how- ever, were not admitted by his contemporaries. He also first defined acids and alkalies in refer- ence to their action on vegetable colors, and