70 
Pasteur and his Work, 
the peasants. Digestion in the worms was impeded because of the 
presence of these organisms in their intestines, and they were 
generally unhealthy ; while those which spun their cocoons pro- 
duced chrysalides which were little better than pulp. It was thus 
shown that the corpuscles, gaining access with the food into the 
intestinal canal, infested all the body in a few days ; the spots 
on the skin were only the effect of the disease, being somewhat 
allied to the eruption of measles in mankind. Another lot of 
healthy worms fed on untainted mulberry-leaves at the same 
time, remain perfectly free from the malady. This and similar 
experiments were repeated, and varied times without number, so 
as to prove the correctness of the conclusions at which he had 
arrived, and exactly accounted for what took place in the silk- 
worm establishments. From the malady which attacked the 
worms at their birth, destroying a whole cultivation, down 
to the invisible disease that lurked in the cocoon, all was ex- 
plained. But as in these establishments worms were never 
directly affected through food purposely soiled by diseased 
worms, it was asked how they became diseased. Pasteur 
pointed out that, in a cultivation of silkworms in which there were 
diseased ones, these were continually fouling the food by their 
excreta, which the microscope showed to be swarming with these 
corpuscles. This cause of natural contagion was rendered all the 
more effective, because the worms, by the weight of their bodies, 
pressed the excreta against the leaves in crawling over them. By 
mixing these excretions with water, and painting mulberry-leaves 
with them, a single leaf off the latter enabled Pasteur to infect 
as many worms as he liked. Another natural and direct cause 
of infection was inoculation by the hooks on the feet of the 
worms, which, in crawling over each other, wounded their skins ; 
and if these hooks were soiled by infected excretions, or by the 
corpuscles immediately beneath the skin, infection was certain. 
Infection at a distance through the medium of the air <and 
the dust it carries, was an equally well-ascertained fact. It was 
sufficient to sweep the breeding-houses, or shake the hurdles 
on which the silkworms were reared, to raise the dust of dead 
worms and corpusculous excretions, which falling over the 
hurdles of healthy worms, after a time caused the disease to 
appear among them. When quite healthy worms were placed 
in a breeding nursery at a considerable distance from unhealthy 
worms, they, in their turn, became infected. It was observed, 
that in some factories healthy worms were reared, in which the 
year before nearly all had perished. Pasteur explained this by 
showing that dust can only act as an infective matter when it is 
fresh ; corpusculous matter loses its virulence when thoroughly 
dried, and a few weeks are sufficient to bring about such a result. 
