308 Proceedings of Royal Society of Edinburgh. [june 20, 
are to be recognised : the natural processes of vegetable growth and 
decay contribute a large amount of soluble material which is carried 
down by land drainage ; the other source, which supplies both 
soluble and insoluble organic matter, is the drainage of towns where 
a flushing system of sewage disposal obtains, and where its products 
are poured into rivers. There is an important difference between 
the organic material derived from these two sources. This differ- 
ence does not depend on an original diversity of character between 
animal and vegetable waste products ; for though such a diversity is 
doubtless recognisable by delicate tests, it is too fine a point to found 
a generalisation upon for any practical purpose. But the difference 
that does obtain is due to the disparate stage of waste product 
decomposition in which organic matter from the two sources reaches 
a river. In both cases the disintegration is carried on by the 
analytical action of micro-organisms, and finally results in the pro- 
duction of such simpler substances as binary compounds of C, H, 
N, and 0. In the vegetable waste products from land drainage 
this process is far advanced, and the resulting materials are in a 
soluble and much simplified form by the time they reach the river 
water ; whereas in the animal and vegetable waste products present 
in sewage the process is only commencing, and in places where the 
drains are flushed into a river, additional insoluble matter may be 
introduced, and may pass far down the stream before even the first 
stage of its disintegration takes place, rendering it soluble, and thus 
in a position for the completion of the process. It remains then to 
inquire, in regard to the disintegration of organic matter, whether 
the same microbes are present throughout the whole process, or one 
group of ferments is replaced by another in correspondence with the 
constantly changing chemical equations that express the several 
stages by which it advances. That the latter is the case, analogy 
strongly suggests, and experimental results go far to prove. As an 
analogous case, that of the fermentation of sugar offers a good 
example, where the first stage of the process is due to the action of 
the Torula cerevisece , and concludes with the formation of alcohol ; 
the further stage of acetous fermentation being produced by a 
distinct species, the Mycoderma aceti. A similar case is that of the 
lactic fermentation of milk, where Bacterium lactis initiates the 
process, changing lactose into lactic acid, at which point the 
