WATER chlorination practice has evolved, during the past fifty years, from a relatively crude application to a highly efficient one. Such a steady process of progressive change is characteristic not only of this practice, but of all applications of scientific knowledge. The results of chlorination can be evaluated on the basis of the chemical and bacteriological results which have been accomplished by its use, and, for this purpose, it will be helpful to review briefly the history (1) of this method of water treatment.