388 THE CAUSES OF THE xi 



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 be- 

 sides these, M. Pasteur found also an immense 

 number of other organic substances such as spores 

 of fungi, which had been floating about in the air 

 and had got caged in this way. 



He went farther, and said to himself, " If these 

 really are the things that give rise to the appear- 

 ance of spontaneous generation, I ought to be able 

 to take a ball of this dusted gun-cotton and put it 

 into one of my vessels, containing that boiled in- 

 fusion which has been kept away from the air, and 

 in which no infusoria are at present developed, and 

 then, if I am right, the introduction of this gun- 

 cotton will give rise to organisms." 



Accordingly, he took one of these vessels of in- 

 fusion, which had been kept eighteen months, 

 without the least appearance of life in it, and by a 

 most ingenious contrivance, he managed to break 

 it open and introduce such a ball of gun-cotton, 

 without allowing the infusion or the cotton ball to 

 come into contact with any air but that which had 

 been subjected to a red heat, and in twenty-four 

 hours he had the satisfaction of finding all the in- 

 dications of what had been hitherto called spon- 



