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.
It was noticed, however, that the students who finished these courses, while well versed in the principles of the science, were not able to apply them in practice. Towards the middle of the century, therefore, a radical change in the system of instruction was inaugurated. The student was put to work and taught to question nature for himself. The universities of France and Germany were equipped with working desks where students of chemistry put into practice at once the principles of the science which they heard elucidated in the lecture room. Cooke, at Harvard, was the chief apostle of the laboratory method in this country, and this method of instruction has now spread, until even the high and grammar schools have their chemical laboratories.
In our universities, students may now begin their chemical studies associated with laboratory practice in the first year of their course, and continue it to the end. Graduates of such courses are not only grounded in the theories of chemistry, but are thoroughly familiar with its practice. Under this system, coupled with the demand for chemical services in every branch of industry, the number of trained chemists has speedily increased. At this time (1899) there are more than four thousand trained chemists in the United States.