There are two experimental paths by which the facts relating to organic impurity in water may be approached, the chemical and the biological; and there are two aspects in which these facts may be regarded, the catalytic and the fermentative. The series of observations made in following the one path is the necessary complement of that met with in the other. The path of chemical investigation has been well cleared, and is easily traversed; that of biological inquiry is still beset by many impediments, and is as yet by no means a safe one. It is the object of many recent researches, and of this paper, to lessen these difficulties. The pollution of water with organic substances is inevitably associated with the presence of those organisms whose function it is in the economy of nature to disintegrate such materials. These organisms in their turn are dependent upon organic matter for their continued existence; the Bacteriaceae to which they belong being distinguished from allied forms by an entire absence of chlorophyll, which obliges them to feed only on organic substances. The relation of these two factors of pollution, each to each, is therefore a necessary one; and due regard must in all cases be paid to each in determining the degree of pollution that their coexistence implies