ELECTRICITY 57 



Cohn's experiment. Ho passed a current through a nutrient solution, 

 and ascertained that when two or more battery cells were used for 

 over 12 hours the medium would not support bacterial life. That the 

 bacteria were not directly affected was proved by the fact that when 

 they were inoculated afterwards into another medium, normal growth 

 was produced. It was suggested that this method could be used in 

 the purification of sewage, and tentative experiments have been made 

 in this direction. The current is generated by a large dynamo 

 machine, and into the water which is to be purified are dipped two 

 large electrodes. The water flows between the electrodes and becomes 

 subjected to the influence of the current. According to Fermi, a 

 current of O'5-l'O ampfere reduced the number of germs to 

 between -^^ and -^^^ of the original number. Experiments were 

 made by Burci and Frascani to ascertain the eff'ect of an electric 

 current apart from the effect of changes in the medium. The 

 bacteria were dried on a pad of glass wool at 37° C, and then dipped 

 into a mercury trough. As mercury is a good conductor of elec- 

 tricity, a current could be passed through this liquid, and its 

 effect on the bacteria observed by subsequently inoculating them 

 into a nutrient medium to see whether they were still capable of 

 germination. It was found that the direct effect of an electric 

 current is to kill bacteria. 



Still more conclusive results were obtained by the experiments of 

 Spilker and Gottstein. The bacterial material was placed in a flask 

 round which was coiled a wire. An induction current was then passed 

 through the wire, when, of course the bacteria, being in the electric 

 field, became subject to electric influences. The organism experi- 

 mented upon was Micrococcus prodigiosus. It was found that this 

 form was killed off when a current of 2 '5 amperes was passed for 24 

 hours through 2.50 c.c. of the nutrient material containing the bacteria. 

 Cocci as a class are less resistant than bacilli, so that a stronger current 

 was necessary to kill off the latter, and, when they were in the form of 

 spores, it was found impossible to destroy completely all the spores in 

 a nutrient medium, although the number could be diminished. An 

 electric treatment is, therefore, not sufficient to sterilise milk, and its 

 expense would render the method impractical when a simpler and less 

 expensive method, that of applying heat, eflTects the same result. The 

 only application that has been made of electricity in connection with 

 nutrient fluids is in the artificial maturing of wines and cognacs to 

 produce certain flavours. This, however, does not depend upon bac- 

 terial effects, but upon the production of chemical changes which 



