Conspicuous achievements, marking the closing years of the century, have been the discovery of the methods whereby organic nitrogen is rendered suitable for plant food, and atmospheric nitrogen fixed and rendered available by leguminous plants. In the first instance, it has been established that organic nitrogen in the soil can only be utilized by plants after it has been oxidized by bacterial action. In the case of leguminous plants, nitrogen is rendered available for nutrition by means of bacteria inhabiting nodules in the roots of the legumes. These two great discoveries have proved of incalculable benefit to practical agriculture. Chemical science in its relations to agriculture has shown that the fertility of the soil may be conserved and increased, while the magnitude of the crops harvested is sustained or augmented. Thus, no matter how rapid may be the increase of population, agricultural chemistry will provide abundant food. VIII. GRAPHIC CHEMISTRY. [Illustration: LOUIS JACQUES DAGUERRE.] The honor of discovering that prints could be made by the action of light on certain salts, such as those of silver, belongs to Daguerre, in 1839. The fundamental principle of graphic chemistry is that metallic salts, sensitive to the light, when in contact with organic matter, suffer a complete or partial reduction and are rendered insoluble. The intensity of the reduction is measured exactly by the intensity of the light. When light is reflected from any object capable of producing different degrees of intensity, as from the hair and face of a man, the reduction of the metal is greatest by the light from that portion of the physiognomy which gives the greatest reflection. Thus, when the unreduced metallic salt is washed out, a permanent record, the negative, of the object is left. It is a long step from the first daguerreotype to the modern photograph, but the principle of the process has remained unchanged. Photographs in natural colors have of late years been obtained. One method is by interposing a film of metallic mercury behind the sensitive plate which must be transparent. The reflected rays of light, having different wave lengths, precipitate the metal in superimposed films, corresponding to the wave or half-wave length. When a negative thus formed is seen by reflected light, the emergent rays from the superimposed films acting as mirrors are transformed into the original colors of the photographed object. The various methods of printing by heliotypes, photolithographs, photogravures, etc., are illustrations of the application of graphic chemistry to the arts. IX. DIDACTIC CHEMISTRY. The lectures of Davy and Faraday in England, of Wöhler and Liebig in Germany, of Chevreul and Dumas in France, and of Silliman (1779–1864) in this country, made the study of chemistry attractive and easy during the early part of the century.