1897J MICROSCOPICAL JOURNAL. 303 



The danger consists not in the quantity of such organ- 

 isms but in their power of growth under given conditions. 

 If capable of living in water, they may infect an entire 

 stream instead of disappearing by processes of dilution 

 within a few rods. Unlike chemical poisons, they have 

 no fixed poisonous dose. The smallest possible inocula- 

 tion may prove fatal through the power of self-propaga- 

 tion which they possess. If, on the other hand, their 

 growth be hindered by unfavorable temperature, moisture 

 or food supply, they may become harmless no matter 

 what their quantity. It is true that they have chemical 

 effects, originating substances known as toxines, some of 

 which are deadly poisons, but they themselves depend 

 upon possession of life for the modes of activity which 

 they exhibit. Throughout it is a question of vitality 

 under particular surroundings. 



Typhoid fever, cholera and certain forms of dysentery 

 ai'e the chief diseases whose infection it is generally 

 admitted can live in water. In addition, about ten years 

 ago, the writer came to the conclusion that the term 

 malaria, signifying bad air, is a misnomer, and that 

 diseases of this class are very largely, if not exclusively, 

 conveyed in water. Towns taking their public water- 

 supply from ponds or streams having distinctly malarial 

 surroundings have become subject to such fevers although 

 previously free from them. 



The manner of spreading of the diseases which have 

 been named originates two classes of dangers. If water 

 be taken from the vicinity of human habitations there is 

 liability to contamination from excreta washed into the 

 |)ond or stream used as a source of supply, or, in the case 

 of wells, the strong action of powerful pumps may origi- 

 nate a rapid flow unJergi'ound extending many hundreds 

 of feet and carrying impurities through coarse gravel or 

 open crevices in the soil. That this is the fact appears 

 from the manner in which ordinary wells at a (-oiisidera- 



