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were not destroyed by boiling. It was supposed that heating
them hot enough to cook an egg would be sufficient, but ex-
perience showed that that was not sufficient. There are some
organisms the spores of which will not be destroyed by an
hour's boiling; the spores of several of the house molds will
grow after we have had them in boiling water for two hours,
so persistent is the life in these little things. Some addi-
tional means had to be devised. It was found, however, that
boiling would destroy the growing plants. Then we learned
to set the media away for so many hours until the spores
that were in it had started their growth, and then boil it
again, and by repeating that three times we would be able to
destroy all living matter and get a media that was sterile, or
that had no life in it. In this media we could plant and grow
pure this variety or that, so that to-day one who has be-
come skilled in the handling of micro-organisms can grow
distinct species with certainty.
Conditions of Growth.
The conditions of the grozvth of micro-organisms are very
important. Micro-organisms grozv upon organic matter, the
higher plants grozv upon inorganic matter. Now, this will ap-
pear to be a sharp line of cUstinction, and so it is, as between
the micro-organisms and the higher plants in general, but if
we follow closely from the lower to the higher we will find
them running into each other so closely that all line of dis-
tinction seems to be banished. Among the higher plants,
even, we have what are known as the fly catchers that seem
to gather animal food and use it in some degree, and among

some of the very low plants we find those that live entirely
upon inorganic material, so that the line of distinction seems
to be blotted out whenever we try to follow from the higher
to the lower or the lower to the higher, and yet the micro-
organisms, as we find them in Nature, grow entirely upon
organic material. Their function in the economy of the uni-
verse is to destroy organic material.
Moisture is necessary to the grozvth of micro-organisms.
Many varieties of micro-organisms will simply cease to grow
if they become dry, and when moisture is supplied to them
they will grow again; no amount of drying seems to injure
them, while other species of micro-organisms are destroyed
by drying. It seems that the bacillus of typhoid fever is de-

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