152
THE PRESIDENTIAL ADDRESS.
nitrates in pure chalk rock that the supposed sewage contamination
might have been derived from antediluvian animals. Experience
showed only too clearly, as, for instance, at Caterham, that water
might receive the most deadly pollution without appreciably affecting
the chemical constitution.
Chemistry also failed to explain the purification of rivers after
some measure of pollution ; in fact, some chemists denied that
purification could take place at all, because water shaken in
bottles or running along glass troughs was not purified.
A real grasp of the whole problem only became possible when
the study of micro-organisms had thrown a light both on the real
mode of communication of water-borne disease, and of the changes
that go on in water itself.
The first attempt in the direction of Bacteriological study applied
to water seems to have been made by the late Dr. Angus Smith, who
called attention to the great differences shown by samples of water
when mixed at a moderate temperature with a sufficient quantity of
gelatine to set into a jelly on cooling. Some samples remain for a
considerable time without change ; others in a very short time begin
to liquefy in more or less numerous points, and very soon show signs
of decomposition. Dr. Smith left it, however, to other workers to
carry out these investigations with more accurate methods ; and in
this country we owe to Dr. Percy Frankland's well-known investi-
gations a clear recognition of the importance of this method of the
biological study of water. Fuller investigations have developed
methods of cultivating the micro-organisms present in water in such
a way as to select particular ones from a number of others, and so
develop their colonies as to enable the expert to identify them with
certainty. Unless this is done we only get the information that there
are an alarming number of organisms in the water we drink, which
may be very deadly to the human frame, or which may be as harmless
as mites in cheese, or, possibly, even beneficial.
A very practical result of these researches has been to show the
enormous value of the system of filtration through sand which has
been adopted by the Water Companies as the result of engineering
experience and skill, and which now proves to have excellences the
very need for which was not known when the process was worked
out. The entire separation of all organisms and germs from water is
a most difficult result to obtain by mere filtration ; even with unglazed
porcelain, though for a time the liquid passes through sterilized,