AND THE HEALING ART 503 



of bacteria, wliich was so important that I must devote a few words to its descrip- 

 tion. With a view to the successful study of the habits and effects of any par- 

 ticular microbe outside the living bod}^ it is essential that it should be present 

 unmixed in the medium in which it is cultivated. It can be readih- 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 ingenious attempts made to effect this object had often proved entire 

 failures. Koch, however, b}' 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 become a solid 

 mass when cold, it should remain fluid though reduced 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 laver 

 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 intermingling, 

 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 the 

 great progress effected by the new method. It was soon introduced into his 

 own institute and other laboratories throughout the world ; and it has immensely 

 facilitated bacteriological study. 



One fruit of it in Koch's own hands was the discovery of the microbe of 

 cholera in India, whither he went to stud\' the disease. This organism was 

 termed by Koch from its curved form the ' comma bacillus ', and by the French 

 the cholera vibrio. Great doubts were for a long time felt regarding this dis- 

 covery. Several other kinds of bacteria were found of the same shape, some of 

 them producing very similar appearances in culture media. But bacteriologists 

 are now universally agreed that, although various other conditions are necessary 

 to the production of an attack of cholera besides the mere ]n-esence of the vibrio, 

 yet it is the essential materies morbi ; and it is by the aid of the diagnosis which 

 its presence in any case of true cholera enables the bacteriologist to make, that 

 threatened invasions of this awful disease have of late years been S(^ successfully 

 repelled from our shores. If bacteriology had done notliing more for us than 

 this, it might well have earned our gratitude. 



