Page:The Harvard Classics Vol. 51; Lectures.djvu/127

 Rh of meat in a glass flask; next the flask was immersed in boiling water until the contents had been thoroughly heated throughout, and then the behavior of the solution on standing was observed. After thorough heating no signs of putrefaction were revealed to the eye or to the nose; no living things were ever visible in the solution under the microscope. But on admitting the air to the flasks putrefaction soon set in and thus proved that the fault was not with the effect of heat upon what is to-day called the culture medium, but that putrefaction had not previously occurred simply because all germs originally present had been killed by heat; sterilized, in short.

The early nineteenth century made two highly important new contributions to the old problem : the view that all living things are made up of cells as their ultimate structural elements; and, secondly, acquaintance with various digestive ferments contained in liquids like the gastric juice, which are now known to be cell free, yet are capable of bringing about processes resembling fermentation. The latter discovery led at a later date to the distinction between organized (living) and unorganized ferments.

Out of the cell theory have grown the wonderful modern sciences of embryology, largely through the efforts of K. E. von Baer, and pathology, in which Rudolf Virchow has a similar position. The study of ferments and fermentation, and of simple chemical agents which can produce like changes, has led to many new problems and to new methods of attacking old ones.

The chemical aspects of fermentation have a special historical importance because they are especially associated with Pasteur's discoveries. Trained as a chemist, he applied the exact methods of physical science to the biological problem, and solved what had been thought by many insoluble. The studies of Pasteur convinced the scientific world that life as we know it never originates spontaneously, that minute living organisms—microbes, germs, bacteria—are far more active agents in this world than had been guessed. Such organisms turned out to be the essential factors in fermentation of