complished at 70° C. in thirty seconds a complete pasteurization, instead of in twenty minutes, i. e., about 90 per cent, of the bacteria were killed, whilst the diphtheria, typhoid, tubercle and pus organisms were destroyed in the same remarkably short period of time, viz., thirty seconds. This will serve to illustrate how the physical agent of heat may be employed, as well as the sensitiveness of bacteria to heat when it is adequately employed.
Bacteria are much more sensitive to high than to low temperatures, and it is possible to proceed much further downwards than upwards in the scale of temperature, without impairing their vitality. Some will even multiply at zero, whilst others will remain alive when frozen under ordinary conditions.
I will conclude this discourse by briefly referring to experiments recently made with most remarkable results upon the influence of low temperatures on bacterial life. The experiments were conducted at the suggestion of Sir James Crichton-Browne and Professor Dewar. The necessary facilities were most kindly given at the Royal Institution, and the experiments were conducted under the personal supervision of Professor Dewar. The action of liquid air on bacteria was first tested. A typical series of bacteria was employed for this purpose, possessing varying degrees of resistance to external agents. The bacteria were first simultaneously exposed to the temperature of liquid air for twenty hours (about—190° C). In no instance could any impairment of the vitality of the organisms be detected as regards their growth of functional activities. This was strikingly illustrated in the case of the phosphorescent organisms tested. The cells emit light which is apparently produced by a chemical process of intracellular oxidation, and the phenomenon ceases with the cessation of their activity. These organisms, therefore, furnished a very happy test of the influence of low temperatures on vital phenomena. These organisms when cooled down in liquid air became non-luminous, but on re-thawing the luminosity returned with unimpaired vigor as the cells renewed their activity. The sudden cessation and rapid renewal of the luminous properties of the cells, despite the extreme changes of temperature, was remarkable and striking. In further experiments the organisms were subjected to the temperature of liquid air for seven days. The results were again nil. On re-thawing the organisms renewed their life processes with unimpaired vigor. We had not yet succeeded in reaching the limits of vitality. Professor Dewar kindly afforded the opportunity of submitting the organisms to the temperature of liquid hydrogen—about—250° C. The same series of organisms was employed, and again the result was nil. This temperature is only 21° above that of the absolute zero, a temperature at which, on our present theoretical conceptions, molecular movement
