36 METHOD OF DESTROYING ALG^E IN WATER SUPPLIES. 



suggested in the preceding section in the case of acid and soft waters 

 may be employed that is, precipitate the copper by some soluble 

 hydroxide or carbonate. This somewhat complicates the treatment, 

 as it will be necessary to determine from the character of the water 

 the amount of copper necessary to produce a solution of 1 to 100,000, 

 as well as to estimate how much of the hydroxide or carbonate should 

 be added. That such work be conducted under the constant and direct 

 supervision of competent authorities is even more important than when 

 treating for algal contamination. 



COMPARISON OF EFFECT OF OTHER DISINFECTANTS. 



A comparison of the efl'ect of copper sulphate with certain other 

 substances commonly used as disinfectants is instructive, and gives 

 some idea of the great toxicity of this metal. Mercuric chloride (cor- 

 rosive sublimate) is slightly more fatal to typhoid and cholera than 

 copper sulphate acting at a lower temperature and in a shorter length 

 of time. Carbolic acid, one hundred times as strong as the dilution 

 found to be effective for copper sulphate, and acting eight times as 

 long, failed to kill. The same is true of formalin used between fifteen 

 and twenty times the strength of a 1 to 100,000 solution. Using one 

 thousand times the amount of citric acid that would be used of copper 

 sulphate produces death. Thymol is effective in six hours when used 

 in a solution of 1 to 5,000, and naphthalene is five times weaker. 



COLLOIDAL, SOLUTIONS. 



The preceding experiments have dealt with copper in solution as the 

 salt, of some acid. The effect upon water of metallic copper surfaces, 

 producing the so-called colloidal solution of copper, deserves especial 

 mention. As Niigeli, Galeotti, and Israel and Klingman have abun- 

 dantly demonstrated, the slight amounts of copper thus brought into 

 solution are highly toxic to many forms of algae and bacteria. 



The experiments carried on in this laboratory^ show that it is 

 undoubtedly possible to exterminate Uroglena and some forms of 

 Spirogyra by suspending in the water copper foil sufficient to give 

 an area of about 1 sq. cm. to each 100 cc. of water. This would not 

 be a practicable method of treating a reservoir, but it suggests the 

 possibility of sheet copper being used as a preventive of pollution. 

 By suspending large sheets of this metal at the intake of a reservoir, 

 it is probable that conditions would be rendered sufficiently antago- 

 nistic to algal growth to maintain the sterility of a reservoir after 

 it had once been thoroughly cleansed of polluting forms. It would, 

 of course, be necessary to keep such copper sheets clean in order to 

 prevent a reduction of the toxic action due to the formation of an 

 insoluble or slimy coating on its surface. It is possible that some 



