TUE BRANCHES' OF CHEMISTRY. The facts of chemistry have been grouped in a variety of ways, either in the interests of researeh or ac cording to their usefulness in connection with kindred sciences or with the arts. Hence such titles as Animal, Vegetable, Medical, Astronom ical, Metallurgical Chemistry. etc.. which in a general way explain themselves. Chemistry proper may he considered as comprising the fol lowing four branches: analytical, descriptive, general, and applied. Analytical chemistry may he defined as the art of determining the compo sition of substances; under the names of tech nical analysis, physiological analysis, etc., many of its methods form an important part of ap plied chemistry. Descriptive chemistry deals with the chemical and physical characteristics of substances; it forms a record of the properties of substances, which are arranged, for conve nience of reference or for didacti• purposes. in accordance with the principles of general chem istry. The two great subdivisions of descriptive chemistry are inorganic and organic chemistry. the latter dealing with the compounds of car bon, the former with those of all the other elements. qcncral chemistry includes theoreti cal and physical chemistry. which are usually treated together: theoretical chemistry com prises the laws of the composition and chem ical behavior of compounds; physical chemistry treats of the physical properties of compounds, of homogeneous mixtures. and of the physical phenomena (thermal, electrical, etc.) :lemur paving the transformations of substances in general. Applied chemistry comprises all the facts and methods of chemistry that find prac tical employment. The most important sub divisions of this branch are: (1) Biological chemistry, including the chemical facts eon met ed with physiological and pathological phenom ena in animals and plants; (2) agricultural chemistry, which deals with problems of rural economy; and (3) industrial. technological, or practical ehemistry, which deals with the uses of chemistry in the arts and manufactures.
TnE METnons OF CHEMICAL PHILOSOPHY. Like any other seience, chemistry may use two dif ferent ways in discovering and demonstrating its general principles. On the one hand—and this is the surest way—a principle may be in duced front a large number of experimental it is then nothing but the state ment of a general fact, and is termed an cm, piricul Thus, the principle of the conserva tion of matter is an empirical law. Perhaps this law may suggest itself a priori; but a- a law of science it has been induced from facts es tablished by the balance. On the other hand, there are problems which cannot be attacked by experiment. Thus, the problem of the ulti mate structure of matter lies far beyond our power of direct observation: yet it is intimately concocted with the correlation of substances, and therefore chemistry is compelled to consider it for purely practical reasons. ln eases of this nature, chemistry, like any other science. and like speculative philosophy, makes some plaus ible assumption. termed a hypothesis. Like speculative philosophy, it develops the hypothe sis, combines it, if necessary, with other assump tions. and thus builds up a theory. But at this point. where speculative research readies its two plus ultra. the work of the scientist really begins. The general principles forming part of the theory are busily applied to phenomena capa ble of direct observation, and then, if their cor rectness is Unheated by actual experiment, they become theoretical lairs. A scientific theory has for its object, first, to correlate seemingly different facts, and, secondly. to throw light on the road of investigation and lead to the estab lishment of new facts. Thus, the atomic theory of chemistry has correlated the various chemical substances with regard to their composition and constitution, and it has revealed the possible existence of innumerable compounds many of which have since been actually prepared—an achievement not unlike the discovery of Neptune by theoretieal astronomy.