14 CLINICAL BACTERIOLOGY AND HiEMATOLOGY 



of condensed water which might otherwise fall into the funnel) 

 or in a warm (but not hot) oven, and left at a temperature of about 

 40° C, until the process is complete. 



The gelatin which is made by the above process is sufficiently 

 clear for most purposes. A more sightly medium may be made 

 by clarification of the above by white of egg. To the medium 

 (after neutralization, but before filtration) add the white of one 

 egg for each 250 or 300 c.c. of fluid, and shake thoroughly. Now 

 boil in the steamer for half an hour, and filter as before. 



Test-tubes are filled with gelatin just in the same way as with 

 broth, and the process must be carried out quickly to avoid 

 solidification of the medium. Some of the test-tubes are allowed 

 to cool in the vertical position, others lying in a sloping position, 

 so that the upper surface of the gelatin forms an ellipse some 

 3 inches long. The former tubes are inoculated by driving a 

 straight platinum needle charged with the material containing 

 the bacteria into the gelatin in the axis of the tube ; cultures 

 made in this way are called " stab cultures." The gelatin 

 "slopes" are inoculated by drawing the charged needle along the 

 surface of the medium, care being taken not to plough it up; 

 cultures made in this way are called " stroke cultures." 



Agar, or Agar- Agar, is the name given to the dried strips of a 

 Japanese seaweed. It forms a jelly which differs from that con- 

 taining gelatin in that it melts at a higher temperature ; nutrient 

 agar, as used in the laboratory, melts just below the boiling-point 

 of water and sets at about 40° C. This is an advantage in the 

 cultivation of most pathogenic bacteria, for these grow (as a rule) 

 best at or near the temperature of the body, the temperature to 

 which they are exposed under natural circumstances ; and at this 

 temperature gelatin would melt. It is not liquefied or digested by 

 any known organism, and this is an advantage in ordinary work. 

 Where the liquefying power has to be determined, special cultures 

 are made on gelatin or blood-serum, and under ordinary circum- 

 stances it is a disadvantage to have part of the medium in a fluid 

 state, with consequent mixing of all the colonies of organisms 

 present. Agar is somewhat difficult to prepare unless the 

 practitioner has an autoclave, and may be bought with advantage. 

 But the following method is not very difficult, and, as agar 

 is perhaps the most generally useful of all media, should be 

 learnt. 



Requisites. — i Broth. 



