Page:Popular Science Monthly Volume 10.djvu/668

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called back to the experiments with which the inquiry this year began. As already stated, it was begun in September, and, leaving out the earlier experiments, I passed on to October 30th. I have now to bring your attention back to the earlier experiments performed in the laboratory. They were suggested by the ingenious investigations of Dr. William Roberts, of Manchester, and by the subsequent investigations of a man to whom we are indebted more than to any other for the knowledge we possess of the different species of those small organisms that we call bacteria; I refer to Prof. Cohn, of Breslau. Let me say that I entertain the very highest opinion of the intelligence and ability with which Dr. Roberts has carried out these experiments; they are in the highest degree creditable to him. This is the experiment to which I refer: Some chopped hay is put into a little can; it is raised to a temperature of 100° to 120°; it is kept for three hours, then poured off and filtered. Last year, we found that hay thus treated was sterilized by five minutes' boiling. I mean that, when it is exposed to the air that has this floating matter removed from it, it never shows any sign of microscopic life. Now, if you examine this natural hay-infusion with litmus-paper, you will find that it turns the litmus-paper red, showing that it is an acid infusion. Dr. Roberts found that acid infusions could be easily sterilized, and his mode of proceeding will be evident from the figure that I have here drawn. He took a vessel with an open neck at the top (A, Fig. 2), and filled it two-thirds full with the infusion he wanted to operate upon; he then stuffed the neck with cotton-wool, and sealed it hermetically with a spirit-lamp above the plug of cotton-wool (B, Fig. 2); he then placed it in a vessel containing cold water, and he gradually raised the water to a state of ebullition and maintained the boiling temperature for any required time. In that way he avoided all commotion, all evaporation, all ebullition in the infusion. After he had placed the tube in this condition in the water, and subjected it to a boiling temperature for any required time, he took it out and simply filed across the neck and broke it off, as I do with this one (C, Fig. 2). Here you have the infusion practically exposed to the atmosphere. The plug intervenes to prevent the entrance of dust, and still allows an interchange between the air of the bulb and the air outside. When Dr. Roberts took this acid infusion and neutralized it by the addition of caustic potash, he found it to possess the most extraordinary power of resistance to heat; he found that, in some cases, it required more than two hours to reduce this infusion to sterility; he also found that, in a particular case, it actually required no less than three hours' boiling to produce this effect. This was very different indeed from the results that I had obtained last year. I made many experiments with hay-infusion, and in every case we sterilized it by five minutes' boiling. I was led to take up the subject this year through the emphatic manner in which Prof. Cohn corroborated the results of Dr. Roberts.