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. X. CHEMISTRY OF FERMENTATION. Our knowledge of fermentation and bacterial action is practically all comprised in the achievements of the nineteenth century. Prior to this time it was known that fermentation took place, but its causes and character were wholly mysterious. The great work of Pasteur (1859) resulted in the fact that fermentations were chiefly caused by the activity of living cells, which have the capacity of reproduction. The most common form of fermentation is that whereby sugar is converted into alcohol and carbon dioxide. The name of the organism that produces this change is _saccharomyces cerevisiae_. Another class of fermentation is seen in the process of digestion. This species of fermentation is typified by the action of sprouted barley on starch, whereby the starch is converted into sugar. The active principle of the saliva, ptyalin, has the same property, and when starchy bodies are masticated, a part, at least, of the starch which they contain is converted into sugar. The active principle of malt is known as diastase, and this, as well as ptyalin, belongs to a class of ferments which are incapable of reproduction. [Illustration: LOUIS PASTEUR.]