792 METHODS OF INVESTIGATING BACTERIA. 



sixty minutes in the current of steam. The mixtures con- 

 taining gelatine often lose their power of solidification if 

 the heat has been continued for too long a time ; hence 

 for these mixtures we employ discontinuous heating, i.e., 

 the gelatine mixture is placed on the first day in the 

 steam for only five minutes; it is then kept till next day 

 at a temperature of 20 to 30 C., so that any living 

 spores which have remained behind may germinate ; 

 it is then exposed to the steam for five minutes on 

 the second day, and this is repeated on the third day. 

 In the case of blood serum, which must be obtained 

 clear and transparent, temperatures of only 55 to 

 60 C. are employed, and for five or six days in succes- 

 sion the material is exposed to this temperature for 

 several hours (see p. 663). 



Special M etlw ds of Preparing some of the Nutrient Substrata. 



Nutrient sub- Potatoes are immersed for half-an-hour in a solution 



strata. 



Potatoes. of corrosive sublimate in order to kill the resisting spores 

 present in the particles of earth which adhere to the 

 outside of the potato ; they are then washed with 

 sterilised water, i nd cooked for thirty to forty minutes 

 in the steaming apparatus. They are then placed to 

 cool in a sterilised and covered vessel, and when they 

 are cold they are cut by means of a knife which has been 

 previously heate'd, and is still warm ; the cultivation is 

 then spread over the freshly cut surface. 



Meat infusion. Meat infusion and nutrient jelly. One kilo of good 

 beef, free from fat, is minced and placed in two litres of 

 water ; after it has been kept for twenty- four hours at 

 15 to 20 C., the fluid is drained off, and the solid 

 material is well squeezed ; the infusion is then placed in 

 flasks and boiled for one hour in the steaming apparatus* 

 and then filtered. One per cent, of peptone and half 

 per cent, of common salt is then added to the filtrate, 

 which is neutralised with soda solution ; the mixture is 

 again boiled and filtered, and then introduced into the 

 cultivation vessels ; these vessels are again sterilised in 



Nutrient the steaming apparatus for twenty to thirty minutes. If 

 a solid substratum is required we add, in addition to the 

 peptone and common salt, 5 to 10 per cent, of gelatine, 



