In spite of the great amount of experimental work carried out. by the alchemists, and the large number of new facts discovered by them, their writings were so obscured by mysticism, exaggeration and deceit, that little real progress was made toward a more accurate understanding of the nature of chemical compounds which might be utilized in constructing a more satisfactory method of classification. No attempts were made to determine the actual constituents of compounds, for it was assumed that in the formation of a compound the original substances were annihilated and an entirely new substance created. Hence the only classification in vogue was a rough grouping of substances according to their physical properties, or apparent outward resemblance, and many of our common names are reminders of this bygone empirical method. Thus, olive oil and other vegetable and animal oils were grouped with oil of vitriol and oleum tartari (deliquesced potassium carbonate); spirit of wine (alcohol) with fuming spirit of Libavius (stannic chloride), spirit of hartshorn (ammonium hydroxide solution) and spirit of nitre (nitric acid); butter with butter of antimony (antimony trichloride) and other semi-solid metallic chlorides. Colorless solids, soluble in water and of characteristic well marked taste, were all classed as "salts," and this group thus included sugar.

The goal toward which the alchemists strove was the philosopher's stone, the grand elixir or the magisterium, as it was variously called, whose virtues were such that it could not only transmute baser metals into silver and gold, but could also prolong life indefinitely. As the claims concerning the transmutation of metals were increasingly discredited and the trickery and deception of the alchemists exposed, more investigators directed their attention toward the second great function of the philosopher's stone, the prolongation of life, and many compounds were discovered of considerable therapeutic value. Great interest was aroused by these investigations, and Paracelsus finally announced that "the object of chemistry is not to make gold but to prepare medicines." Thus, in the first half of the sixteenth century, chemistry began to develop in a new direction, at first not far removed from alchemy, but gradually diverging from it more and more widely, and approaching closer and closer

to medicine, until the coalescence of the two sciences appeared practically complete. And thus arose the period of iatro chemistry, when chemistry, which had long been looked upon as a valuable helpmeet to medicine, came to be regarded as the basis of the entire. medical art.

Although in this period the chief development was again along the mineral side, probably because of the relatively greater simplicity and stability of these preparations, still no little organic investigation was conducted and a number of new compounds were added to the science. Little progress was made in gaining a truer insight into the character of chemical compounds, and hence no important changes in classification appear. Paracelsus himself, the founder of the iatro-chemical school, adopted Basil Valentine's three elements (sulfur, mercury and salt) as the basis of his doctrines.

By the middle of the seventeenth century, chemistry awakened to the fact that it had a destiny of its own to realize, struggled to its feet and, refusing longer to be supported by other sciences, started forward, to be sure rather unsteadily and uncertainly at first, but with the firm determination to do something for itself.

The history of chemistry proper begins with Robert Boyle about 1660, who taught that its main object was the determination of the composition of matter. Through his labors, and those of Rouelle and others, the terms "element" and "chemical compound" were more fully explained and appreciated; nevertheless many of their colleagues still adhered to the old alchemical or even the Aristotelian elements. Kopp, in his "Geschichte der Chemie," gives an excellent picture of the epoch-marking effect of Boyle's ideas:

"What a contrast is exhibited between the ancient idea of the cause of difference in various forms of matter and that which obtained at the time of Boyle! If we consider these two opposite conceptions historically, and the transition from the one to the other, they appear like two totally dissimilar pictures; but, like dissolving views, changing the one into the other by slow degrees. In the first place we have the Aristotelian idea, according to which, matter itself devoid of properties, becomes endowed with characteristic qualities by the addition of properties, and forms, when invested with these properties, the various substances known in nature; then this idea passes gradually into that of the alchemists, but becomes confused in the transition, inasmuch as the differences of physical condition and properties are no

longer regarded as the only causes of varieties in substances; the difference in chemical properties receives more attention, the existence of elements, the producers of such properties is assumed; and thus the path is prepared which leads to the idea of chemical composition. Then we see the Aristotelian theory gradually becoming indistinct, whilst the idea of the importance of the chemical deportment and composition of bodies assumes prominence, and at last we see clearly that the differences between the substances which nature presents to us in such overpowering numbers, or which we have ourselves formed artificially, depend upon differences in their chemical composition. The idea of chemical composition, which makes its first appearance indistinctly in the history of the chemistry of the Middle Ages, now forms the foundation of the science."

The most important and interesting problem at this time, and the one upon which most attention was focused, was the chemistry of combustion. Attempts to explain the phenomena of combustion finally led to the phlogiston theory of Stahl, which dominated the science from the end of the seventeenth through the eighteenth century.

In 1675, Nicolas Léméry published his "Cours de Chimie," which soon became one of the most popular textbooks of the time and passed through thirteen editions during its author's lifetime. In it he divided all natural substances into mineral, vegetable, and animal; including in the second group plants, resins, gums, fungi, fruits, acids, juices, flowers, mosses, manna and honey; and under the third heading describing the various parts of animal bodies. This classification was quite generally adopted, and thus arose a distinct separation of mineral chemistry from the chemistry of substances occurring in plants and animals. The phlogistonists had previously opposed any such subdivision, contending that the differences observed depended upon a variation in the composition of the bodies classed under the three heads. So Becher, in 1669, argued that the same elements occur in the three natural kingdoms, but that they are combined in a simpler manner in mineral substances than in vegetable or animal. Stahl, in 1702, asserted that in vegetable as well as in animal substances the watery and combustible principles predominated, and that these ultimate constituents made their appearance when the organic compound was heated out of contact with air, water and combustible charcoal being

formed. These ideas were successfully combated by Boyle, who had shown, as early as 1661, in his "Sceptical Chymist," that the application of heat leads to quite different results depending upon whether air is present or not, and that the various residues thus obtained are unlike.

Many organic substances were discovered during this phlogiston period, but their real composition (even qualitative) remained unrecognized. For example, it was assumed that the ultimate constituents of alcohol were oil and water, or a combustible and a mercurial principle. By far the greater number of the investigations recorded were still in the inorganic field, probably for reasons already given, and also because it had not as yet been possible to prepare organic compounds synthetically. While, as has been said, many authors adopted Léméry's method of separating mineral, vegetable and animal substances, others still adhered to the old system of grouping together all acids (sulfuric with lactic, tartaric, etc.), all salts, etc.

Boyle's influence was soon effective in directing a closer scrutiny of the composition of compounds, and gradually the true elements. were isolated and studied.

The discovery of the composition of carbonic acid gas by Lavoisier in 1775, and that of water by Cavendish, showed the presence of carbon and hydrogen in alcohol (1784). Lavoisier, having established the true principle upon which combustion depends, analyzed various organic substances and came to the conclusion that vegetable substances were composed generally of carbon, hydrogen and oxygen, while animal substances contained also nitrogen and occasionally phosphorus. He did not distinguish organic chemistry as a special branch of the science, or define it as "the chemistry of the compound radicals." He discussed all acids together, subdividing them into mineral, vegetable and animal.

Macquer, who was professor of medicine in the University of Paris, and a contemporary of Lavoisier, in his "Elements of the Theory and Practise of Chymistry" (English translation of 1775) discusses mineral, vegetable and animal oils together, and in the separate sections of his work devoted to vegetable and animal.

chemistry divides the subject according to the method of treatment employed to obtain the substance rather than according to the character of the substance itself. Thus we have as the main headings. "Operations on unfermented vegetables," "Operations on fermented vegetable substances," and "Operations on animal substances."

Fourcroy (about 1790), however, in his well-known text-book, makes a clean-cut division, placing the vegetable acids in the section dealing with the vegetable kingdom, and the animal products all under the animal kingdom.

It should be noted that at this time carbon was supposed to exist as such in plants and animals. So Chaptal, in 1791, says:

"Carbone exists ready formed in vegetables. It may be cleared of all the volatile and oily principles by distillation, and, by subsequent washing in pure water, it may be deprived of all the salts which are mixed and confounded with it."

In Thomson's "System of Chemistry" (third edition, 1807), marsh gas and olefiant gas are discussed with the element carbon, but the other carbon compounds are scattered under various headings where they are mixed in with inorganic substances.

In the text-books and treatises on chemistry at this period it was customary to combine mineralogy and geology with the mineral part, botany with the vegetable section, and physiology with the portion dealing with animal chemistry, while occasionally physics received as much space as chemistry in the introductory chapters.

The ætherin theory of Dumas and Boullay, propounded by them in 1815, and later adopted by Berzelius, was an adaptation of the early theories concerning the composition of organic compounds (by which they were supposed to consist of an aqueous and a combustible principle) to new conditions. In their theory, many derivatives of alcohol were regarded as compounds of C2H, (to which Berzelius had given the name "ætherin"), in the same way that ammonium salts are derived from NH3: