382 ON THE LACTIC FERMENTATION 
operating upon the Bactertwm lactis as a poison or sedative, as it has been shown 
to act on the bacterium of the butyric fermentation. The inquiry has assumed 
larger proportions than I had anticipated: but I may briefly mention here some 
of the facts ascertained. I found that when I imitated Pasteur’s experiment 
with the fermenting saccharine solution and exposed milk from a dairy in a thin 
layer to the atmosphere with arrangements for avoiding evaporation, the souring 
of the milk was retarded to a much greater degree than I had observed it in 
the comparatively thick layer in the liqueur-glasses of the former experiment. 
This effect became still more marked when oxygen was substituted for 
atmospheric air: but the unexpected fact was also elicited that carbonic-acid 
gas is still more potent than oxygen in retarding the lactic fermentation. The 
relations which these gases bear to the growth of the Bactertum lactis must be 
reserved for future consideration. 
I have introduced into this diagram a representation of the Torula Cerevisiae 
side by side with the Bacterium lactis, drawn on the same scale enlarged from 
camera-lucida sketches (the sketches are reproduced on their original scale in 
Plate XIV, Figs. a to 12). My object in so doing is to bring out the remarkable 
contrast between them in point of magnitude. This is especially striking if we 
compare the Torula Cerevisiae with the Bacter1um lactis as it was found after 
growing for three days in milk diluted with 1200 parts of boiled water (Fig. 11). 
Here they have assumed a condition so minute that the individuals of every 
pair do not equal in size the smaller granules of the torula. Being motionless, 
I could hardly have recognized them as bacteria except from their mode of 
grouping and the circumstances under which they occurred. Yet Bacteriwm 
lactis they undoubtedly were, and one such barely visible granule introduced 
into a gallon of uncontaminated milk would lead within three days to its con- 
version into a solid mass. It is a ferment just as potent as the torula in spite 
of its extreme minuteness ; and this circumstance, it seems to me, has an inter- 
esting bearing upon pathology. For it cannot be said to be an unlikely thing 
that other organisms may exist as much smaller than the Bacterrwm lactis as 
that bacterium is than the torula. But if this be the case, such organisms 
must be entirely beyond the range of human vision aided by the best micro- 
scopes that we possess. Seeing, then, that the hypothesis which I make cannot 
be regarded as extravagant, it will not do for us to say, because we cannot see 
under the microscope any organisms in a given infective liquid, that they cer- 
tainly do not exist, and that therefore we must abandon the idea to which 
analogy might otherwise lead us, that the virus concerned may be of the nature 
of an organism. We see, I say, from the comparison of these two sketches, 
that it is far from being impossible that there may exist ultra-microscopic 
