CHEMICAL AGENCIES 117 



in proportion to the length of exposure ; she found that they might lie dormant 

 for as long as 426 days. Her conclusions are that the reason why the spores 

 resist heat is because they are characterized by the possession of an imperme- 

 able membrane, which is also the cause of their delayed germination. The 

 thicker the membrane, the more resistant is it to heat, and the longer does the 

 organism take to develop. 



The evidence in favour of the second view does not appear to us to be as con- 

 vincing as that in favour of the first. Eijkman's figures, given in Table 4, are 

 very striking, and, assuming their general validity, it is difficult to avoid con- 

 cluding that the effect of heat is to increase the lag period of such organisms as 

 remain viable. That the escape of the few is dependent on the possession of a 

 relatively impermeable cell membrane is quite possible, but it fails to explain 

 why in Eijkman's experiments before heating all organisms developed in 3 

 days, whereas after heating some failed to develop for 15 days. 



Though sub -lethal heat may delay germination of spores, the opposite effect has been 

 recorded by some workers. Christian (1931a, b), for example, working with an aerobic 

 spore-bearing bacillus isolated from tainted milk, found that germination appeared to be 

 stimulated by heating the spores to 100° C. for 30 minutes at the time of inoculation. 

 Evans and Curran (1943) also observed that heating spores of some species of aerobic 

 spore-bearing bacilli for 10 minutes at 85° C. often stimulated germination. In general, 

 mild heating followed by 3 hours' incubation led to about the same degree of germination 

 as 24 hours' incubation without pre-heating. 



Chemical Agencies 



Distilled Water. — The evidence concerning the action of distilled water on the 

 viability of bacteria is most conflicting. Spores are undoubtedly able to survive 

 for a long time ; thus Koch foimd that spores of the anthrax bacillus remained 

 alive for more than 90 days. But with vegetative organisms it is otherwise. 

 Some workers have found that they will survive for weeks, others that they are 

 destroyed in a few hours. Such confusion can be explained only by difEerences 

 in technique. One such difference of primary importance is the nature of the 

 vessel from which the water is distilled. When a metallic still is used, traces of 

 the metal are carried over into the distillate and undoubtedly exercise a deleterious 

 effect on the bacteria. Ficker (1898) found, for instance, that water containing 

 copper sulphate in a dilution of 1/50,000,000 was sufficient to kill cholera vibrios 

 in 1 hour, while Hoder (1932) found that distilled water containing 1 part of copper 

 in 10 million sterilized a suspension of Ps. pyocyanea in 2 hours. 



But even those who state that they used glass-distilled water have obtained 

 varied results. One reason for this discrepancy may lie in the number of bacteria 

 inoculated. In this connection some striking figures are reported by Ficker (1898). 

 In one experiment he seeded pure glass-distilled water with 60,000,000 cholera 

 vibrios per ml., and found that they remained viable for several months. In 

 another experiment, in which he reduced his inoculum to 10,000 per ml., the 

 organisms were nearly all dead in 2 hours. He explains such a difference on the 

 assumption that the inoculation of large numbers of organisms into distilled water 

 converts this into a dilute nutrient medium, no longer possessing the essential 

 purity of the initial menstruum. The H-ion concentration of the water may also 

 be an important factor. Thus Winslow and Falk (1923) give the following figures, 

 compiled from no fewer than seventy-nine tests : 



