648 THE POPULAR SCIENCE MONTHLY. 



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 labo- 

 ratory. 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 earned 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, wdien it is ex- 

 posed 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 tempera- 

 ture 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 tem- 

 perature 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 inter- 

 change 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' boil- 

 ing 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' boil- 

 ing. I was led to take up the subject this year through the emphatic 

 manner in which Prof. Cohn corroborated the results of Dr. Roberts. 



