THE GERM-THEORY OF DISEASE. 579 



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 recog- 

 nized, 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 ves- 

 sel, 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 con- 

 tinue 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 them- 

 selves. 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 bac- 

 terial 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 putrefac- 

 tion, 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 



