92 LOUIS PASTEUR 



in the long narrow neck. When this was added, 

 living forms appeared in abundance. What was 

 this something? 



Pasteur affirmed that it was germs floating in 

 the air, and he set himself to demonstrate that 

 germs actually do float in the air. To show this, 

 he caused air to be drawn through a tube, plugged 

 with gun cotton to act as a sort of filter. After a 

 given volume of air had been drawn through the 

 tube, the gun cotton was dissolved in alcohol and 

 ether. A residue was deposited which, under the 

 microscope, was seen to be composed of many kinds 

 of particles, among which were bodies indistin- 

 guishable from the spores of mold, the cysts of 

 infusorians, and various kinds of bacteria. That 

 the cotton actually contained the germs of organ- 

 isms was shown by another experiment. Taking a 

 flask of sterile infusions, Pasteur, under precautions 

 to keep out the entrance of matter from the out- 

 side, placed a small bit of the cotton in the neck 

 but without allowing it to come into contact with 

 the liquid; then the neck of the flask was sealed. 

 So long as the cotton remained in the neck of the 

 flask, the liquid kept clear. After fifteen days, or 

 a month or longer, the flask was tilted so as to bring 

 the liquid in contact with the cotton. Soon the 



