PHYSICAL AGENTS AND BACTERIAL LIFE. 247 



complished at 70° C. in thirty seconds a complete pasteurization, in- 

 stead 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 tempera- 

 tures, 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 Eoyal Institu- 

 tion, and the experiments were conducted under the personal super- 

 vision of Professor Dewar. The action of liquid air on bacteria was 

 first tested. A typical series of bacteria was employed for this pur- 

 pose, 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 intra- 

 cellular 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 re- 

 newal of the luminous properties of the cells, despite the extreme 

 changes of temperature, was remarkable and striking. In further ex- 

 periments 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 



