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Proceedings of Royal Society of Edinburgh . [june 2Q ; 
2. Spring Water . — Water from deep wells is always, in the first 
instance, nearly free from the presence of microbes; that from shallow 
wells may be seriously contaminated with sewage matter, and may 
be loaded with organisms. Deep wells in the chalk at times supply 
water rich in organic matter, but which only yields evidence of 
microbic activity after it has stood for a time in contact with the 
air. In such cases it is not unfrequently more crowded with 
organisms even than river water, — a condition probably due to the 
smaller number of species present in such cases ; for thus the struggle 
of opposing vital requirements is avoided, and the total sum of pos- 
sible individual life thereby increased. 
3. River Water . — Glacier water is free from microbes, but in all 
other cases river water contains a larger or smaller number of micro- 
organisms, depending on the relative amount of organic matter that 
it holds in solution. The nature of the land drained by a river, the 
presence or absence of direct sewage infection, and the speed of 
its flow, are the chief conditions affecting the purity of its water. 
As will be shown in the second part of this paper, the popular sen- 
timent in favour of rapid and tumultuous rivers as a source of 
domestic supply is probably based on a misconception, and is com- 
pletely at variance with sound deductions from the facts of the case. 
4. Sewage. — The rich supplies of organic matter here present permit 
of an enormous development of microbes, but the fermentative activity 
thus established is of a duration inversely proportional to its inten- 
sity. After a maximum development occurring on the third or 
fourth day, a rapid decrease is observed in the number of microbes, 
so that in the course of a week to ten days there may be a smaller 
number in a sample of sewage than in a stored specimen of river 
water (Bischof). In this case the large amount of food material is 
rapidly exhausted by the disproportionate production of microbe 
life ; and when the supply comes to an end, the death-rate amongst 
the microbes is for a time excessive. 
We must now turn to the tests employed in determining the 
number and varieties of microbes present in water, and inquire into 
their accuracy and reliability. Various methods have been suggested, 
some giving quantitative, others qualitative results. It is obvious 
that a method giving reliable information on both points is desirable, 
since the total number of microbes present in a specimen of water 
