138 TRANSACTIONS OF THE PLYMOUTH INSTITUTION. 
to the formation of alcohol, and another to the production of 
glucose. Hence the great difference between English and German 
beers ; the former being much more alcoholic, the latter containing 
more glucose, and fat-forming materials. And again another form 
will cause alcohol to take up oxygen, and so convert it into vinegar 
and water. Another causes the conversion of sugar of milk into 
lactic acid, and so turns milk sour. These illustrations will show 
that we are surrounded on every hand by these minute organisms, 
and explain what an enormous influence they have on us. 
III. The saprogenic or putrefactive bacteria produce changes 
very similar to fermentation, in complex organic substances. 
By the action of these bacteria in the putrefactive substances 
poisonous alkaloids are formed, which are called ptomaines. 
IV. Pathogenic, or disease-producing organisms. I have before 
referred to the method of proof that these organisms really do 
produce disease, and are not merely the concomitants thereof. 
This is clearly proved when, as in the bacillus of anthrax the 
cultivation has been passed through successive generations, and then 
by reproducing the disease in a suitable animal. The malignancy 
of anthrax is very well known. The disease has been reproduced 
in cattle by their grazing on land under which animals which have 
died from anthrax are buried. The spores can be kept for years ; 
for instance on silk threads soaked in a cultivation of anthrax on 
potato — the anthrax grows readily on potato — and then if the 
silk threads are soaked in a mixture of the potato with sterilised 
water, they become impregnated with the spores. The minutest 
portion introduced under the skin of an animal will produce 
anthrax years afterwards. 
Forty years ago the anthrax bacillus was discovered in the 
blood of animals suffering from splenic fever. Thirteen years 
afterward, Davaine showed that the disease could be produced 
by inoculation; but it was not until 1877, when Professor Koch 
introduced the methods of cultivation before referred to, that the 
bacillus was isolated, and it was proved that it, and it alone, 
produced the disease. One difficulty in the way of proving 
the dependence of a disease on a given bacillus — is that bacilli 
which may be found in healthy secretions such as the saliva — 
will produce disease in the lower animals ; so that unless precisely 
the same disease is produced, the microbes cannot be said to be 
the cause. 
