PHYSICAL AGENTS AND BACTERIAL LIFE. 239 



still remain of primary importance in relation to many industrial 

 processes in which they play a vital part. It will be seen, therefore, 

 that their biology presents many points of interest to scientific workers 

 generally. Their study as factors that ultimately concern us really 

 began with Pasteur's researches upon fermentation. The subject of 

 this evening's discourse, the effect of physical agents on bacterial life, 

 is important not merely as a purely biological question, though this 

 phase is of considerable interest, but also on account of the facts I have 

 already indicated, viz., that micro-organisms fulfil such an important 

 function in the processes of nature, in industrial operations and in 

 connection with the health of man and animals. It depends largely on 

 the physical conditions to be met with in nature whether they die or 

 remain inactive. Further, the conditions favoring one organism may 

 be fatal to another, or an adaptability may be brought about to unusual 

 conditions for their life. To the technologist the effect of physical 

 agents in this respect is of importance, as a knowledge of their mode of 

 action will guide him to the means to be employed for utilizing the 

 micro-organisms to the best advantage in processes of fermentation. 

 The subject is of peculiar interest to those who are engaged in com- 

 bating disease, as a knowledge of the physical agents that favor or 

 retard bacterial life will furnish indications for the preventive measures 

 to be adopted. With a suitable soil and an adequate temperature 

 the propagation of bacteria proceeds with great rapidity. If the 

 primary conditions of soil and an adequate temperature are not present, 

 the organisms will not multiply, they remain quiescent or they 

 die. The surface layers of the soil harbor the vast majority of the 

 bacteria, and constitute the great storehouse in nature for these forms 

 of life. They lessen in number in the deeper layers of the soil, and 

 few or none are to be met with at a depth of 8-10 feet. As a matter 

 of fact, the soil is a most efficient bacterial filter, and the majority of 

 the bacteria are retained in its surface layers and are to be met with 

 there. In the surface soil, most bacteria find the necessary physical 

 conditions for their growth, and may be said to exist there under natural 

 conditions. It is in the surface soil that their main scavenging func- 

 tions are performed. In the deeper layers, the absence of air and the 

 temperature conditions prove inimical to most forms. 



Amongst pathogenic bacteria the organisms of lockjaw and of 

 malignant oedema appear to be eminently inhabitants of the soil. As 

 an indication of the richness of the surface soil in bacteria, I may 

 mention that 1 gramme of surface soil may contain from several hun- 

 dred thousand to as many as several millions of bacteria. The air 

 is poorest in bacteria. The favoring physical conditions to be met 

 with in the soil are not present in the air. Though bacteria are to be 

 met with in the air. they are not multiplying forms, as is the case in 



