12 CLINICAL BACTERIOLOGY AND H^MATOLOGY 



water, but if this is not at hand the whole apparatus (flask 

 and funnel) may be placed in the steam sterilizer (the lid being- 

 kept off to avoid the drops 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 suffi- 

 ciently 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 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 inocu- 

 lated 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 cul- 

 tures." 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 containing- g'elatin in that it melts at a higher tempera- 

 ture ; 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 g'elatin 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 



