teems with them. No terrestrial waters are free from them. They form a part of the atmospheric dust, and are deposited upon all objects exposed to the air. It is difficult to demonstrate this truth directly with the microscope, for in the dry state bacteria are not readily recognized, especially when few in number. But we can easily detect their presence by their power of multiplication. We need but provide a suitable soil. An infusion of almost any animal or vegetable substance will suffice—meat-broth, for instance—though not all bacteria will grow in the same soil. Such a fluid when freshly prepared and filtered, is clear as crystal, and remains so if well boiled and kept in a closed vessel, for boiling destroys any germs that may be present, while the access of others is prevented by closure of the flask. But as soon as we sow in this fluid a single bacterium, it multiplies to such an extent that within a day the fluid is turbid from the presence of myriads of microscopic forms. For this purpose we can throw in any terrestrial object which has not been heated previously, or we can expose the fluid to the dust of the air. Air which has lost its dust by subsidence or filtration through cotton has not the power of starting bacterial life in a soil devoid of germs. Of course, the most certain way of filling our flask with bacteria is to introduce into it a drop from another fluid previously teeming with them.
In a suitable soil each bacterium grows and then divides into two young bacteria, it may be within less than an hour, which progeny continue the work of their ancestor. At this rate a single germ, if not stinted for food, can produce over fifteen million of its kind within twenty-four hours! More astounding even seems the calculation that one microscopic being, some forty billion of which can not weigh over one grain, might grow to the terrific mass of eight hundred tons within three days, were there but room and food for this growth!
During their growth the bacteria live upon the fluid, as all other plants do upon their soil. Characteristic, however, of bacteria-growth is the decomposition of any complex organic substances in the fluid to an extent entirely disproportionate to the weight of the bacteria themselves. This destructive action occurs wherever bacteria exist, be it in the experimental fluid, or in the solid animal or vegetable refuse where they are ordinarily found. It constitutes, in fact, rotting or putrefaction. The processes of decomposition of organic substances coming under the head of putrefaction are entirely the effect of bacterial life. Any influence, like heat, which kills the bacteria, arrests the putrefaction, and the latter does not set in again until other living bacteria gain access to the substance in question. Without bacteria, no putrefaction can occur, though bacteria can exist without putrefaction, in case there is no substance on hand which they can decompose.
No error has retarded more the progress of the germ-theory than the false belief that the bacteria of putrefaction are identical with the germs of disease. The most contradictory results were obtained in