311 
1887.] Mr A. W. Hare on the Purity of Water. 
gives an indication of the quantity of. organic matter necessarily pre- 
sent for the support of so much life ; and the special forms present 
and their proportion to one another, is no less important, from the 
indication thus given, as the sequel will show, of the state of decom- 
position at which such organic material has arrived. 
(a) Of the purely quantitative biological tests that by l( dilution ” 
may be mentioned, employed by Fol and Dunant. It consists in 
enormously diluting the sample to be tested with germless water in 
a known proportion, and in inoculating a large number of culture 
glasses with equal quantities of this mixture. The proportion of the 
culture glasses remaining sterile to those showing microbe life affords 
a basis for calculating the number of germs in the original specimen. 
This method is inexact ; it labours under the twofold disadvantage 
of depending on perfect mechanical mixing under great difficulties, 
and of the certainty that the culture fluid used could not suit the re- 
quirements of every microbe present; some, therefore, would not grow 
in it, and would be omitted from the calculation. When the number 
thus obtained is multiplied by the number of dilutions previously 
carried out, the omissions thus made will be multiplied, and the re- 
sulting error in the calculated total most serious. 
(b) Of the purely qualitative methods may be mentioned that by 
“ fractional cultivation,” successfully employed by Lister in separat- 
ing species from one another. It is, however, so laborious as to be 
inapplicable for practical purposes, although it was primarily instru- 
mental in establishing the important scientific principle of specificity 
amongst microbes. 
(c) Another method is that proposed by Dupre, in which the 
nature of organic impurities present is determined by observing 
changes in the aeration of water, the gases absorbed and given off 
giving an index of the amount of vital action occurring in the speci- 
men investigated. It must be seriously doubted whether this can 
ever be developed into an accurate method of observation : the 
factors of sewage contamination are so variable under varied condi- 
tions, that a uniformity of results is scarcely to be hoped for ; while 
there is room for so many fallacies in this method, that it would 
require confirmation from others before its results could be accepted. 
(d) By far the best method of determining the number and 
varieties of microbes in water is that introduced by Koch. It con- 
