56 OUTLINES OF BACTEEIOLOGY 



These results indicate that the vegetative cells of bacteria are killed 

 equally as rapid at 80° C. as at 100° C. and that in each case an 

 exposure of 15 seconds is sufficient to effect their destruction. 



With regard to the resistance to heat of the spores of the Coccaceae, 

 the author has obtained the following result with the spores of the 

 Sarcinae ureae : 



A. at 80° C. 



1 hour + , \\ hours + , 1 J hours + , If hours + , 2 hours - . 



B. at 100° C. 



2 mins. + , 3 mins. + , 3 J mins. - , 4 mins. - , 5 mins. - , 6 mins. - . 

 Hence the spores at 100° 0. and at 80° C. have a power of resistance 

 equal to that of the less resistant kinds among the Bacteriaceae. 



The amount of resistance to heat of any particular species is not 

 always constant. If tw^o cultures of the same species, derived from 

 different sources, be examined, some variability in this respect is sure 

 to be found. Also, just as a gardener may in course of time obtain a 

 new variety by carefully selecting forms which show the required 

 characteristics in a most pronounced form for breeding, so in the 

 same way it is possible to "breed" cultures which can stand more 

 heat than the normal cultures. There appears to be no doubt also 

 that nature does the same by a process of natural selection, by 

 selecting from species with more resistant membranes, so that in 

 course of time a variety is produced which is more resistant to heat 

 than the parent species from which it was derived. This is the 

 more probable because bacteria are amongst the most plastic of 

 organisms. Apart from this, even in the different cultures of the 

 same species, all of which have been derived from the same source, 

 the resistance to heat depends on the age of the spores, their dryness, 

 and probably on other factors unknown as yet. This variability is 

 probably connected with differences in the thickness and density of 

 the outer spore coats. Older spores in a dried condition are the 

 more resistant, younger spores in a moist condition being more easily 

 killed. 



§ 2. ELECTRICITY. 



When a current of electricity is passed through a nutrient medium 

 containing bacteria, the inimical effect may be twofold. In the first 

 place products may be formed by the action of the current on the 

 medium which are detrimental to bacterial life, and in the second place 

 the current may affect the bacteria directly. The first effect is seen in 



