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DUST AND DISEASE. 293

tion. These constituted his “lot temoign,” his standard of
comparison. On the 16th of April, 1868, he thus infected
thirty worms. Up to the 23d they remained quite well.
On the 25th they seemed well, but on that day corpuscles
were found in the intestines of two of them. They first
form in the tunic of the intestine. On the 2’7th, or eleven
days after the infected repast, two fresh worms were exam-
ined, and not only was the intestinal canal found in each
case invaded, but the silk-organ itself was found charged
with corpuscles. On the 28th, the twenty-six remaining
worms were covered by the black spots of pébrine. On the
30th, the difference of size between the infected and non-
infected worms was very striking, the sick worms being not
more than two-thirds of the size of the healthy ones. On
the 2d of May, a worm which had just finished its fourth
moulting was examined. Its whole body was so filled with
corpuscles as to excite astonishment that it could live.
The disease advanced, the worms died and were examined,
and onthe 11th of May only six out of the thirty remained.
They were the strongest of the lot, but, on being searched,
they also were found charged with corpuscles. Not one of
the thirty worms had escaped; a single corpusculous meal
had poisoned them all. The standard lot, on the contrary,
spun their fine cocoons, and two only of their moths were
found to contain any trace of corpuscles, which had, doubt-
less, been introduced during the rearing of the worms.

As his acquaintance with the subject increased, Pas-
teur’s desire for precision augmented, and he finally gives
the growing number of corpuscles seen in the field of his
microscope from day to day. After a contagious repast,
the number of worms containing the parasite gradually
augmented until finally it became cent. per cent. The
number of corpuscles would at the same time rise from 0 to
1, to 10, to 100, and sometimes even to 1,000 or 1,500 for a
single°field of his microscoPe. He then varied the mode

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