360 CHEMISTRY showed that what has been dissolved in the one may be precipitated by adding the other. In applying this to analysis, he made a great step, the so-called dry method of analysis having been previously exclusively used. To this day several reactions first described by Boyle are in common use ; for example, the detection of am- monia by adding lime to the matter containing it, and observing the fumes that are formed in presence of an acid ; also the detection of silver by muriatic acid. He likewise made many ob- servations of affinity. Nicholas L6mery (1645- 1715), a famous lecturer at Paris, deserves mention for his efforts in the diffusion of chem- ical knowledge. As a clear and systematic writer, he was justly celebrated. Another modification of views regarding the elements, which exerted great influence, was expressed by Becher (died 1682). According to him, all inorganic bodies are composed of earthy ingre- dients. There are three elementary earths, the fusible, the combustible, and the mercurial, severally principles of fluidity, combustibility, and volatility. These three earths are present in all metals, combined in different proportions. When combined with water they form the salts, and also a universal acid which is the basis of all acids. He regarded the calcination (oxidation) of metals and combustion in gen- eral as a process of decomposition depending on the expulsion of the combustible earth by means of fire. A simple body incapable of decomposition could not burn, for every body capable of burning must contain within itself a cause of its combustibility. This doctrine was soon adopted by Stahl, and under the name of the phlogiston theory characterized an epoch in the history of the science. Chemistry now first stands out on an equality with the other natural sciences. Its aim is no longer either the making of gold or the curing of disease. A desire to acquire a knowledge of the composi- tion of bodies, to explain the phenomena which accompany their formation and decomposition, and to ascertain what relation exists between their properties and their composition, became now the motive of chemists. Modern chemistry, properly so called, now commenced. The first special problem which attracted attention was the explanation of the phenomena of combus- tion and oxidation. The analogy between these processes had been long observed and often con- sidered. The ancients regarded combustion as dependent upon the separation of fire, appa- rently believing the latter to be something ma- terial. This view was admitted for a long period. The alchemists expressed a similar idea by their figurative sulphur, which was supposed to be expelled when a body burned. For the term sulphur, Becher substituted com- bustible earth, and defined more clearly the idea that it was separated during combustion. The notion that combustion destroys was un- doubted ; something was separated, causing the appearance of flame, while the incombus- tible residue was one of the components of the compound which had been destroyed. In like manner, when a metal was oxidized, the calx (oxide) was considered to be an educt from the metal. The famous phlogiston theory of Stahl (1660-1734) was a more accurate expression of these views. According to him, all combustible bodies must contain one and the same ingredi- ent, to which they owe their common property, combustibility. This combustible matter he calls phlogiston ; its existence, although entirely hypothetical, was regarded by Stahl as being so certain that it was hardly worth while to isolate it, and but few attempts to do so were made by his immediate followers. Later it was thought to be identical with hydrogen. Ex- pressed in the language of the present day, phlogiston may be regarded as the opposite of oxygen. What is now deemed a combination with oxygen was considered by Stahl to be the result of a separation of phlogiston. During combustion phlogiston is expelled, while the other constituents of the compound remain. Charcoal, which leaves little or no residue when burned, was thought to be nearly pure phlogis- ton. In general, the combustibility of bodies was supposed to depend on the proportion of phlogiston which they contain. On examining the residues left by different substances after their phlogiston had been expelled, it was thought that a knowledge of their original con- stitution could be obtained. Experience was thus supposed to teach that phosphorus is a compound of phosphoric acid and phlogiston ; that the metals are composed of calxes and phlogiston. If bodies from which the phlo- giston has been expelled are heated with oth- ers rich in phlogiston, the latter give back phlogiston to the former and the original com- pound is produced. Thus, when a metal calx is heated with charcoal, the original metal is formed. Besides combustibility, Stahl referred color, solubility in acids, and other chemical properties of bodies, to the amount of phlogis- ton contained in them. For example, he show- ed that metals which had been deprived of their phlogiston could no longer unite with sulphur. These ideas were greatly extended by his fol- lowers. Stahl's observations of new facts and upon affinity were numerous and valuable. Al- though unable to free himself entirely from the vague speculations of the alchemists, he adopted in general the idea of elements suggested by Boyle, regarding as such all peculiar constitu- ents of matter which by uniting with each oth- er and with phlogiston form compound bodies. At this period investigations to ascertain what substances should be regarded as elementary commenced. Thus the metal calxes (oxides) were considered to be elements, as were sul- phuric and phosphoric acids, &c. Boerhaave (1668-1738) and Homberg (1652-1715) were especially diffusers of chemical knowlege. The Elementa Chemue of the former, published in 1724, exerted an influence which has probably never been surpassed. His experiments made in order to disprove certain statements of the
