Page:Popular Science Monthly Volume 48.djvu/817

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ENTLEMEN: In selecting a subject for my presidential address I have thought it best to restrict myself to that branch, of biological science with which I am most familiar; and, as a technical paper might prove uninteresting to many of those who constitute my present audience, I have chosen a title for my address which will enable me to speak in a general way of the development of our knowledge relating to the low vegetable organisms known as bacteria, and the practical results which have been the outcome of researches commenced in the first instance solely on account of their scientific interest.

Attention was first prominently called to the bacteria by the investigations relating to spontaneous generation. It was generally believed prior to the researches of Spallanzini, in 1776, that the development of micro-organisms in boiled organic fluids exposed to the air was by heterogenesis. Spallanzini showed by experiment that in some instances putrescible liquids when boiled and kept in hermetically sealed flasks could be preserved indefinitely without undergoing change. But he was not always successful in this experiment. Bastian, and other supporters of the theory of heterogenesis, at a later date, repeated these experiments with similar results, and maintained that when a development of micro-organisms occurred in a boiled fluid contained in a hermetically sealed flask it could only be by spontaneous generation. But Pasteur, in 1860, gave the true explanation of the appearance of living bacteria under such conditions. He proved that when development occurs it is because the organic liquid has not been completely sterilized, and that certain micro-organisms (spores of bacilli) withstand the boiling temperature, especially when they are suspended in a liquid having an alkaline reaction. At the present day this question is regarded as definitely settled, at least so far as known conditions are concerned; and we have an exact experimental knowledge of the thermal death-point of many micro-organisms of this class.

The principal pathogenic bacteria are destroyed at temperatures much below the boiling point of water. Thus, in experiments made by the present speaker in 1885 it was ascertained that