302 THE AMERICAN MONTHLY [Oct. 



detenniiiiitiou of the chemical ingredients held in sus- 

 pension or solution. Elaborate systems of analysis were 

 devised for this piiri)ose, and the quality of the water 

 was judged almost entirely by its chemical reactions. 

 Thus it becomes customary to consider the questions 

 involved from a chemical point of view exclusively. The 

 simple dilution of contained matters of a chemical nature 

 if carried far enough, would make them harmless. Con- 

 sequently large bodies of water were sup^tosed to have a 

 power of self-purification in direct jDroportion to their 

 size. In like manner precipitation, sedimentation, aera- 

 tion and other chemical and mechanic:d processes were 

 supi)Osed to have a purifying effect. The quantity of 

 sewage entering a stream being known, it becomes possi- 

 ble to tell with a good degree of certainty at what distance 

 mingled with such a volume of water, it will become so 

 diluted, diffused and changed as to be unrecognisible by 

 any chemical test. The dose of poisonous matter, if of 

 a chemical nature, ought to be divided and sub-divided 

 to such an extent as to be entirely harmless in the quan- 

 tity of water that any individual would consume. In 

 practice, however, this is not found to be the fact, sewage 

 infection being capable of producing epidemic disease 

 for many miles along a stream entirely out of proportion 

 to any i)Ossible chemical process of diffusion. 



Tlie whole tendency of modern research has been to 

 show that the question as to the spread of disease through 

 the agency of water is liiological rather than chemical. 

 It is the presence of certain living organisms and of the 

 conditions on whicli their continued existence depends 

 that leads to the spread of disease. A single seed may 

 be the means of overspreading an entire continent with 

 some form of luxuriant growth, and so a single disease 

 germ may start an ejddemic, not through any mechanical 

 or chemical process of division or subdivision, but because 

 having life it grows and multiplies. 



