BY R. GREIG-SMITH. 



537 



One of the quadruplicate tests of the last experiment was 

 incubated for a further period of 24 hours to determine if the 

 toxic action continued. 



Experiment v. 



The toxic action continued in the unheated water, while, in 

 the boiled water, the bacteria persisted unaltered. 



The reduction in the numbers of bacteria put into ordinary 

 filtered water points to some injurious factor, and the increased 

 reduction in the same water, when boiled, seems to indicate that 

 the effect is not caused by an absence of organic matter. If, 

 however, we ignore the action of the boiled water, a possible 

 explanation for the destructive effect of raw water might be 

 found in the shock given the bacteria by the transference from a 

 saline media, such as ordinary nutrient agar, to water virtually 

 devoid of saline matter. To test this possibility, the bacteria 

 were grown in sodium chloride-free nutrient agar for several 

 generations, and the experiment again made. 

 Experiment vi. 



The experiment shows that the reason for the destruction of 

 the bacteria is not to be found in the lessened saline content of 

 the media. We are justified, therefore, in concluding that 

 ordinary tap-water contains substances of the nature of bacterio- 

 toxins, the toxicity of which is increased by boiling. 



