ORIGINATION OF LIVING BEINGS. 79 



ought not only to be able to show the germs, but I 

 ought to be able to catch and sow them, and produce 

 the resulting organisms." He, accordingly, constructed 

 a very ingenious apparatus to enable him to accom- 

 plish this trapping of this "germ dust " in the air. He 

 fixed in the window of his room a glass tube, in the 

 centre of which he had placed a ball of gun-cotton, 

 which, as you all know, is ordinary cotton-wool, 

 which, from having been steeped in strong acid, is con- 

 verted into a substance of great explosive power. It 

 is also soluble in alcohol and ether. One end of the 

 glass tube was, of course, open to the external air ; and 

 at the other end of it he placed an aspirator, a con- 

 rivance for causing a current of the external air to 

 pass through the tube. He kept this apparatus going 

 for four-and-twenty hours, and then removed the 

 dusted gun-cotton, and dissolved it in alcohol and 

 ether. He then allowed this to stand for a few 

 hours, and the result was, that a very fine dust was 

 gradually deposited at the bottom of it. That dust, 

 on being transferred to the stage of a microscope, 

 was found to contain an enormous number of starch 

 grains. You know that the materials of our food and 

 the greater portion of plants are composed of starch, 

 and we are constantly making use of it in a variety of 

 ways, so that there is always a quantity of it suspended 

 in the air. It is these starch grains which form 

 many of those bright specks that we see dancing 

 in a ray of light sometimes. But besides these, 

 M. Pasteur found also an immense number of other 

 organic substances such as spores of fungi, which had 



