The living matter of the body is composed of a variety of substances, of which protoplasm may be taken as typical. This is highly complex, and while it can be imitated by the chemist, its living properties have not been reproduced. (See my book on “Life: Its Origin and Nature,” in the present series.) The various secretions and excretions of the body have been studied exhaustively by physiological chemists. _Plants_ have also been studied minutely from a similar point-of-view. A number of important discoveries have resulted from this work, however, and nearly all the essential animal and vegetable substances are at present accessible to artificial synthesis from their very elements. Even protein matter seems to have lost much of its mystery since we have learned from Emil Fischer’s work that amino-acids can be combined in the same way as they occur in protein. Compounds of Amino-acids can be obtained, which show all the main reactions of protein substances. Emil Fischer, of Berlin, was the same chemist who, in 1886, discovered how to prepare grape-sugar from glycerine. A considerable number of plant alkaloids have also been artificially prepared in the course of the last five or six decades. The most important coloring matters of plants--for instance, alizarin and indigotin,--are no longer extracted from plants for technical purposes, but are accessible from the products of coal-tar. CATALYSIS We now come to a remarkable series of chemical phenomena, which have been much studied during the past century, and which have a bearing upon both organic and inorganic chemistry. More than a century ago, it was discovered that certain chemical substances, which will not normally combine with one another, can be made to do so, if another substance is brought into contact with them. This third substance does not in any way enter into the combination, or share in the reaction; its mere presence seems to bring it about. Thus, oxygen and hydrogen may be mixed together; but if a small amount of “platinum black” be introduced, an explosion of the gases at once occurs. Hydroperoxide is rapidly split into oxygen and water when in contact with “platinum black,” etc. These contact-effects are very curious, and have engaged the attention of chemists for a long time. Berzelius is responsible for the term now generally used--_catalysis_. We now speak of catalytic power, catalytic reactions, and so forth.