104 
president’s address. 
May, 1889. 
proportion of nitrate of silver, viz., 1-1,600,000th part, 
abruptly stopped its growth and killed it. The growth could 
not even commence in a silver vessel. Equally fatal were 
one part in 500,000 of corrosive sublimate, one in 8,000 of 
bichloride of platinum, and one in 240 of sulphate of copper. 
Another observer, Duclaux, sowed the spores of the 
Aspergillus on the same liquid nourishing material, in the 
one case with, and in the other case without, the tartaric acid; 
on the first a good crop arose in three days ; on the other 
there was no growth. But, on the other hand, the liquid 
which contained the tartaric acid remained limpid and pure ; 
no Bacteria developed in it: the second was turbid with the 
enormous numbers of Bacteria that crowded it. Add but a 
drop of tartaric acid to the second liquid, and the scene 
changes as by magic ; the spores of the Aspergillus which 
have hitherto remained dormant begin to grow, soon take the 
upper hand, and produce as good a crop as in the other. 
The true Bacteria are influenced almost as easily by the 
character of the circumstances in which they exist. They 
feed upon the substances that surround them, appropriating 
some of the elements themselves, and setting free the 
others to enter into new combinations. The result of their 
action is in the main akin to that which is called fermenta¬ 
tion. In fact, Duclaux defines fermentation as “ chemical 
transformations which dissolved substances undergo, under 
the influence of organisms, always devoid of chlorophyll, 
which develop and live in the interior of the liquid which is 
fermenting.” 
To show the kind of change which Bacteria can work we 
may instance the transformation of sugar into lactic acid, of 
lactic acid into butyric acid, of alcohol into acetic acid, of 
urea into carbonate of ammonia, of ammonia into nitrates, 
and of albumen into peptones. There can be no doubt that 
some of them play a very important part in the process of 
digestion, especially in the stomachs of herbivorous animals. 
Others produce colouring matters, red, blue, green, yellow, 
purple, pink, violet, brown, and, in fact, every possible colour; 
it is also believed that a microbe is the active agent in the 
production of indigo. 
Putrefaction is now known to be but the result of manv 
simultaneous and diverse fermentations, which go on in the 
decaying substance under the influence of Bacteria and a few 
other simple organisms. The most important fermentation 
is that which the Bacteria are capable of producing in 
complex organic nitrogenous compounds. If a substance, 
highly putrescible under ordinary circumstances, be sterilised 
