detected.

_A Practical Demonstration_

The results in the laboratory being successful, the next step was to
make a practical demonstration of the value of the method. This was
first done in the fall of 1901. At Ben, Va., water cress is grown in
large quantities during the winter, when it is a valuable market crop.
Dams are constructed across a stream in such a manner as to enable the
maintenance of a water level not too high for the growth of plants;
when a freeze is threatened the plants can be flooded. In the cress
beds selected for the experiments the water is obtained from a thermal
spring whose temperature throughout the year is about 70 deg. F. This
temperature is particularly favorable to the growth of "frog spawn."
After the cress was cut for market, the algae frequently developed so
rapidly as to smother the life out of the weakened plants. When this
occurred, the practice was to rake out both water cress and algae and
reset the entire bed. This was not only expensive; half the time it
failed to exterminate the pest. It was, therefore, most desirable to
devise a method of ridding the bed of algal growth without injuring
the cress.

_The Copper-sulphate Method Tested_

Here the copper-sulphate method was put to a practical test. At the
outset a strong solution was sprayed on the algae which coated the
surface of the pond. This only killed the algal growth with which the
particles of copper came in contact and left the main body of algae
unaffected. Then trial was made of dissolving the copper directly in
the water, and the result was most satisfactory. The solution used was
that of 1 part of copper to 50,000,000 parts of water.

Growers need have no trouble in the future. They need have no fear of
employing the method, as the copper solution required for killing the
algae could not possibly injure water cress, provided ordinary care is
used in the work. As to the frequency of treatment required, one or
two applications a year will generally be found sufficient, as this
letter, received from the manager of the Virginia company, goes to
show:

"The 'moss' has given me no trouble at all this winter; in fact, I
have for six months had to resort to the copper sulphate only once....
All the conditions were favorable last fall and early winter for a
riot of 'moss,' but it did not appear at all until just a few days
ago, and then yielded to treatment much more readily than it did when
I first began to use t