Page:Popular Science Monthly Volume 23.djvu/44

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34
THE POPULAR SCIENCE MONTHLY

been exposed to a heat of from 158° to 167°, without being free from germs; for, if we afterward add a quantity of broth which has been sterilized at 230°, the mixture of the two liquids, which, separate, would have continued perfectly limpid, will shortly swarm with bacilli and other organisms. Cohn's liquid can not be fully deprived of active germs till it has been boiled for four hours. If, continues M. Miquel,

PSM V23 D044 Bacillus of the atmosphere magnified 1000 times.jpg
Fig. 5.—Bacillus of the Atmosphere magnified 1,000 diameters.

we add sewer-water that has been heated for several hours in an hermetically sealed retort at 176° to Cohn's liquid which has been sterilized at 230°, and place the mixture in a hot bath, nothing will appear in it even after a month. Apparently it is perfectly free from living-germs. But if a few drops of it are placed in a broth, also fully sterilized, the broth will in a day or two appear full of bacilli.

We must, then, unless we would expose ourselves to grave errors, distinguish between apparent and real sterilization. While beef-broth, neutralized with potash and heated to 230°, will remain sterile for an indefinite time, it is a good plan with other compositions and for particular bacteria to attain a temperature as high as 302.

Heat, unfortunately, modifies the composition of organic liquids,