380 
Micro-organisms and their Action 
within twenty-four hours. Upon microscopic examination, one 
kind of micro-organism only is now observed in the milk. To 
make the matter free from all doubt, it is necessary that quite 
pure cultivations should be used and their effects studied. 
From a colony raised in the first instance, a second gelatine 
plate is inoculated ; after the development of new colonies, a 
third, fourth, and fifth plate is taken, and so on until the possi- 
bility of having anything of the original material left is quite 
out of the question. In one case, a series of seventy-eight 
distinct cultivations was made in this manner, and no alteration 
with regard to shape or effect of the ferment was observed. 
The ferment consists of short cells which are not much longer 
than they are broad, and are generally in pairs with their ends 
joined together. To an unpractised eye they would appear to 
be nearly oval, and a double cell would seem to be notched all 
round at the division ; but upon careful examination under suit- 
able conditions they are found to have parallel sides and only 
slightly rounded ends. There are also somewhat longer rods, in 
which a segmentation into two cells is about to take place. The 
cells are motionless, and from their character they must be 
classed as bacilli. 
In some instances the formation of spores has been observed. 
Upon one end of a single or both ends of a double cell, an 
excrescence can be seen, which is of a very high refractive 
power. When such cells are subjected to the heat of boiling 
water, which suffices to kill the full-grown bacilli, it is found 
that fresh cells are developed in a suitable medium. 
This bacillus can live and develop in certain media without 
undergoing any alteration, and without producing any decom- 
position. As soon, however, as it is introduced into milk, it 
gives rise to the formation of lactic acid and the subsequent 
coagulation of the casein. 
That lactic acid and no other acid is formed has been ascer- 
tained by separating the acid in the form of metallic compounds, 
and determining their character and behaviour. The quantity 
of milk-sugar is diminished, corresponding with the increase of 
lactic acid. ^ 
It is not upon milk exclusively that the bacillus is able to act 
and cause decomposition, but it also sets up lactic fermentation if 
introduced into solutions of milk-sugar, cane-sugar, dextrose, and 
mannitose. It seems, however, that in the case of milk-sugar and 
cane-sugar, a hydration, i.e. an addition of the elements of water 
to those of the sugar, precedes the decomposition. As the action 
of chemical ferments consists chiefly in causing the elements of 
water either to be added to or split off the substance acted 
upon, it has been suggested that, like other organized ferments, 
