16 REPORT—1896. 
know its cause ; and it is impossible to over-estimate the practical value 
of researches such as those to which I am now referring. Among their 
many achievements is what may be fairly regarded as the most important 
discovery ever made in pathology, because it revealed the true nature of 
the disease which causes more sickness and death in the human race than 
any other. It was made by Robert Koch, who greatly distinguished 
himself, when a practitioner in an obscure town in Germany, by the 
remarkable combination of experimental acuteness and skill, chemical 
and optical knowledge and successful micro-photography which he brought 
to bear upon the elucidation of infective diseases of wounds in the lower 
animals; in recognition of which service the enlightened Prussian 
Government at once appointed him to an official position of great impor- 
tance in Berlin. There he conducted various important researches; and 
at the London Congress in 1881 he showed to us for the first time the 
bacillus of tubercle. "Wonderful light was thrown by this discovery upon 
a great group of diseases which had before been rather guessed than known 
to be of allied nature; a precision and efficacy never before possible 
was introduced into their surgical treatment, while the physician became 
guided by new and sure light as regards their diagnosis and prevention. 
At that same London Congress Koch demonstrated to us his ‘ plate 
culture’ of bacteria, which was so important that I must devote a few 
words to its description. With a view to the successful study of the 
habits and effects of any particular microbe outside the living body, it is 
essential that it should be present unmixed in the medium in which it is 
cultivated. It can be readily understood how difficult it must have been 
to isolate any particular micro-organism when it existed mixed, as was 
often the case, with a multitude of other forms. In fact, the various in- 
genious attempts made to effect this object had often proved entire failures. 
Koch, however, by an ingenious procedure converted what had been before 
impossible into a matter of the utmost facility. In the broth or other 
nutrient liquid which was to serve as food for the growing microbe he 
dissolved, by aid of heat, just enough gelatine to ensure that, while it 
should beeome a solid mass when cold, it should remain fluid though re- 
duced in temperature so much as to be incapable of killing living germs. 
To the medium thus partially cooled was added some liquid containing, 
among others, the microbe to be investigated ; and the mixture was 
thoroughly shaken so as to diffuse the bacteria and separate them from 
each other. Some of the liquid was then poured out in a thin layer upon 
a glass plate and allowed to cool so as to assume the solid form. The 
various microbes, fixed in the gelatine and so prevented from inter- 
mingling, proceeded to develop each its special progeny, which in course 
of time showed itself as an opaque speck in the transparent film. Any 
one of such specks could now be removed and transferred to another vessel 
in which the microbe composing it grew in perfect isolation, 
Pasteur was present at this demonstration, and expressed his sense of 
