II] PREPARATION OF CULTURE MATERIAL 33 



blood serum, or hydrocele fluid. Koch, who introduced 

 this mixture, used it for the cultivation of bacteria on solids, 

 to be exposed to the air ; the proportion of gelatine in the 

 mixture was 2 to 3 per cent. But this mixture, although 

 solid at ordinary temperature, does not keep solid iri the 

 incubator, not even at 20° C. I have found that at least 

 7 '5 per cent, of gelatine must be contained in the mixture 

 to keep it solid at 20° to 25° C. Above this last tempera- 

 ture not even 1 1 per cent, gelatine will keep solid. 



Nutrient Gelatme, most useful for the growth of all kinds 

 of bacteria, is prepared in this way : — 



One pound of lean beef is cut up, to it is added one pint 

 of water, and is kept boiUng in the digester or any other 

 vessel for from half to three-quarters of an hour. After 

 having been strained through fine calico it is filtered through 

 paper into a beaker ; bring up by adding water to 600 ccm. ; 

 add to this 60 grams of the finest gold label gelatine cut up 

 in small pieces, 6 grams of peptone, and 6 grams of common 

 salt. Dissolve on waterbath, but do not let the water boil ; 

 neutralise with carbonate of soda or, better, liquor potassae 

 till faintly alkaline ; steam for half an hour, filter by hot filter 

 (see Fig. 5) into a sterile flask plugged with sterile cotton- 

 wool, and bring it up to boiling point, at which it is kept for 

 a few minutes. This can be kept as stock gelatine, or can 

 be decanted at once into sterile test-tubes plugged with 

 sterile cotton-wool. These are steamed on two successive 

 days for 20 minutes each time. Keeps solid up to about 

 25° C. 



Prepared in this manner the nutrient gelatine passes easily 

 and comparatively rapidly through filter paper on hot filter. 



The same 10 per cent, nutrient gelatine can be of course 

 obtained if broth is already made — e.g. broth in a stock 

 flask, by adding the above-named quantities of gelatine, 



D 



