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- 
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