106 
president’s address. 
May, 1889. 
transformation offers one of the most surprising spectacles 
we can imagine of the great results arising from a single 
step. 
That step in advance was the publication, in 1881, of 
Koch’s discovery of the method of cultivating Bacteria upon 
solid nutrient gelatine. Let us consider for a moment the 
effect of this discovery, and the further steps which it renders 
practicable, or even easy, where previously an apparently 
insoluble difficulty had barred the way. In a natural state 
many kinds of Bacteria almost invariably occur together; 
even in cases where we may in all probability hope to get but 
one species, as for instance in the blood of a person who has 
died from some specific disease induced by a species of 
Bacteria (what is now called a bacterian disease), even then 
doubts are not prevented. We may get over all the difficulties 
of excluding other germs from our flasks, our nutrient 
solutions, our lancets, and our needles; we may use but a 
minute drop of the blood to be investigated; but after all, if 
two forms of Bacteria ultimately make their appearance in 
the liquid we are using,. we have no guarantee that they 
came from the same original germ. Or again, when we take 
a drop of the solution, and with it inoculate an animal, if any 
disease makes its appearance in consequence, we can have 
no certaintv to what the disease is to be attributed, since we 
cannot tell that there may not have been a species in the 
liquid that was used equally efficacious (or more so) in 
producing the observed result, as that which we intended to 
introduce. 
But when the solid stratum is used and Koch’s method 
followed, all this uncertainty becomes the plainest and most 
entrancing certainty. When we have obtained, by diluting 
with sterilised water, a drop of fluid in which the bacterial 
germs are few in number, we inoculate with this minute drop 
a few cubic centimetres of liquefied nutrient gelatine, mix the 
whole well together, and then spread it out in a thin layer 
upon a plate of glass. When it becomes solid, each germ 
will almost inevitably be isolated from its neighbours and, 
provided that the species is capable of growing in the sub¬ 
stance and at the temperature we are using, each will proceed 
to divide and redivide in its own characteristic manner, and 
form, in a few hours or days, a little colony visible to the 
naked eye. If the dilution of the germs has been carried to 
a sufficient extent, each of these colonies will be at first 
distinct from the others which grow in its neighbourhood. 
By observing the stages of their growth under a low power 
of the microscope, and rejecting all that are irregular or 
