EELATION OF TATHOGENIC TO SEPTIC BACTERIA. 15 



material (in his case it was a solution of Liebig's meat extract), 

 to mix with it purified and well sterilised and neutralised gela- 

 tine solution in such a proportion that the gelatine would form 

 2 — 3 per cent. Such a nourishing mixture is solid at ordi- 

 nary temperatures, and represents an excellent soil for sowing 

 on or in it the desired species of bacterium in dots or lines ; 

 kept in flat glass dishes or slides the examination with the 

 microscope can be easily carried out from time to time, and it 

 can easily be ascertained how and whether the sown species is 

 making progress, and accidental contaminations can thus be 

 easily detected and removed, all growths, owing to the solid 

 state of the nourishing material", being naturally limited to the 

 spot or line on which the bacterium has been sown. It is 

 necessary to keep the dish or glass in a chamber (under a bell- 

 jar) saturated with moisture. This is, in short, the essence of 

 Koch's method. He maintains that such a gelatine material 

 remains solid at a temperature of 20° — 25° C, sufficiently 

 high for the growth of all species of bacteria. 



All this sounds very excellent, but when one comes to work 

 with it practically one finds that everything is not as- perfect 

 and excellent as one imagines at first. 



As is well known from the researches of Brefeld, Grawitz, 

 Wernich, and others, nourishing material in a solid state, such 

 as gelatine, boiled potato, bread, paste, &c., has been used for 

 the sake of obtaining pure cultivations, and for the sake of 

 easily watching and keeping under control the progress and 

 growth of particular organisms, e.g. Penicillium, Aspergillus, 

 Micrococcus prodigiosus, &c.; but most of these observa- 

 tions were carried on at ordinary temperatures. Koch, how- 

 ever, recommends it, after many observations, in the above 

 form for pure cultivations, even in the incubator, at 20° — 25° 

 C. for all species of bacteria (Micrococcus, Bacterium termo, 

 and various species of baccilli, &c.). 



The first difficulty one has to overcome is to obtain a sterile 

 and neutral clear and limpid gelatine solution. I have tried 

 every obtainable kind of gelatine, in which I was much assisted 

 by Dr. George Maddox, to whom I am under great obligations. 



