ount of dissolved carbon dioxide; if very
small, 5 per cent increase is desirable; if large, 0.5 per cent is
sufficient."

The information in this prescription is to be used in connection with
a table[2] published by the Department of Agriculture. This table
gives the number of parts of water to one part of copper sulphate
necessary to kill the various forms of algae which are listed. The
formulae vary from 1 part of copper to 100,000 parts of water,
necessary to destroy the most resistant and very rare forms (three of
these are listed), to 1 part of copper in 25,000,000 parts of water,
which is a sufficiently strong solution to exterminate _Spirogyra_,
the cress-bed pest. By far the majority of forms do not require a
solution stronger than that of 1 part of copper to 1,000,000 parts of
water.

_What the Agricultural Department is Doing_

It is true that the department is not now holding out, directly, a
helping hand to the owner of a country place, or to the farmer, in
this campaign of purifying drinking water. In the first place, the
greatest good of the greatest number demands that large reservoirs,
which supply a great number of people with drinking water, ought to be
considered first. Such supplies, moreover, are most frequently
contaminated. Where fifty reservoirs were treated last summer, ten
times that number will be "cured" this summer. It will be readily
seen, therefore, that in conducting such a large number of
experiments--considering preliminary reports, prescribing for
treatment, and keeping proper account of results--the department, with
a limited force and limited facilities, has its hands more than full.

More important still, there is an absolute need of the services of
some expert on the ground. While an algologist is a functionary not
generally employed by water companies--in fact, a man trained in the
physiology of algae is difficult to find--nevertheless, it is highly
important, as the department views it, to have the cooeperation of an
expert versed to some extent in the biological examination of drinking
water. In other words, the copper cure is not a "patent medicine,"
with printed directions which any person could follow. Intelligence
and care are absolutely essential in the use of this treatment.
Furthermore, each case must be treated as a distinct and separate
case, as a physician would treat a patient.

_Actual Purification Simple_

Suppose, however, an owner of a country place,