309 
1887 .] Mr A. W. Hare on the Purity of Water. 
Bacillus butyris makes its appearance, and from the lactic produces 
butyric acid. It is difficult to prove distinctly that the same law 
holds true in the disintegration of so complex a compound as 
sewage matter ; but the direct evidence obtained from the investi- 
gation of decomposing animal solutions points to that conclusion. 
In such a substance Bacterium termo and its congeners appear, and 
carry on their special functions of decomposition in a definite 
sequence, one group commencing it labours where another has 
completed its special share in the process. In the sequel, it will 
appear, from a series of observations recently made, that different 
species of microbes preponderate in the different areas of a sewage- 
laden river, and it will be attempted to show how these distinctions 
probably depend on the advance of successive fermentative processes 
from stage to sta go, pari passu with the flow of the river from point 
to point, from its initial area of sewage contamination till it is 
restored to a state of relative purity. In the meantime, however, 
the status of microbes in water of different qualities requires 
attention. For the purpose of description it is convenient to 
differentiate four qualities of water, viz., distilled water, spring 
water, river water, and dilute sewage. 
1. Distilled Water. — The absence of organic matter prevents any 
great development of microbes in this medium. Yet marked 
diversities are found in the behaviour of different species in this 
respect. Whilst it has been shown by Crookes, Tidy, and Odling 
that Bacillus anthracis does not long survive its introduction into 
distilled water, and by P. Frankland that the same is true of Koch’s 
Comma bacillus and of Finkler’s and Prior’s bacillus , yet it has been 
shown that the hardy Bacillus pyocyaneus is capable of surviving 
for a considerable time in such conditions, and that, in the first 
place, it even increases in numbers (P. Frankland). Many of the 
ordinary species present in atmospheric dust are also capable of living 
in distilled water ; hence the necessity known to all bacteriologists of 
keeping distilled water used for microscopic purposes rigidly free from 
direct contact with the air, and of frequently obtaining supplies 
freshly prepared. But these contaminations, though very serious 
where exactitude of microscopic observation is at stake, have no 
immediate hygienic importance, and we may consider distilled water 
at least as an absolutely safe substance for human consumption. 
