ON THE BACTERIA. 153 



the Bacteria may escape observation. To remedy this, various 

 processes of staining have been introduced, for the first suggestion 

 of which we are indebted to Koch. Fortunately all Bacteria and 

 Micrococci have great affinity for aniline colours, and are strongly 

 stained by them. We may easily try this for ourselves. If we 

 take the slightest smear of mucus from the mouth, nose, or any of 

 the openings of the body, and dry it on a slide, we shall not be 

 able to detect anything ; but if we place upon it a drop of aniline 

 violet ink, allow it to remain a minute or two, and then wash it off 

 with a gentle stream of water, we shall find plenty of material for 

 study. 



When the Bacteria are contained in animal tissues, somewhat 

 more complicated processes are necessary. The staining of the 

 whole substance sometimes obscures our view of the Bacteria^ 

 but owing to their superior affinity for the aniline colours, it is 

 found possible, by suitable chemical re-agents, to discharge the 

 colour from the surrounding medium, which may then either be 

 left colourless, or subsequently stained with some other colouring 

 matter, to throw into more vivid contrast the unaffected Bacteria. 

 The way in which different Bacteria are acted upon by different 

 colouring matters and chemical re-agents, seems likely to be of 

 use as a means of diagnosis between varieties. 



Having discovered the existence of Bacteria in a diseased 

 animal, by the microscope, the next process is to cultivate them 

 outside the body of the animal in which they are found, so as to 

 have an opportunity of watching their life-history; and having 

 obtained them perfectly pure, and free from contamination with 

 any of the fluids of the body, to make further experiments as to 

 their effects when introduced by inoculation into the bodies of 

 healthy animals. Cultivation may be carried on either in chemical 

 solutions or in animal broths, which are sterilised by suitable eleva- 

 tions of temperature, in tubes plugged with cotton wool to prevent 

 the accidental introduction of atmospheric germs. Such sterilised 

 fluids are charged with the minutest drop of fluid known to contain 

 the Bacteria, which soon multiply to an enormous extent, and from 

 which any number of successive inoculations may then be made 

 in fresh portions of fluid with the same result. 



In studying the life-history of the Bacteria^ by means of 



