Convinced of the need of earnest work, extensive laboratory experiments were inaugurated. The problem presented was this: the remedy must not only be readily available, but it must be cheap, that advantage may be taken of it by the poorest communities, as well as by those owning large reservoirs. Above all, the remedy must be absolutely harmless to man; the poison used to exterminate algae must not in any way affect the water drinkers. A large number of substances were used in the experiments before the final decision rested with copper sulphate. This salt is very poisonous to algae. On the other hand, copper in solution just strong enough to destroy algal growth could not possibly injure man; in fact, the temporary presence of such a small amount of copper in drinking water could not be 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: