SUGAR- A GAEBLOOD-SER UM. 41 



on agar unless from 3 to 5 per cent, of glycerin is added 

 (tubercle bacillus). The glycerin is added just before the 

 medium is put into the tubes. 



Sugar-agar : From 1 to 5 per cent, of sugar is added to the 

 agar in solution. This medium is used to demonstrate fer- 

 mentation or gas-production. 



Blood-agar : The surface of the medium is smeared with a 

 drop of blood taken from the finger-tip or ear, or from the 

 vein of an animal. This medium is used for cultivating the 

 influenza bacillus. 



Blood-serum : This is made directly from the blood. Quite 

 a number of organisms will grow on blood-serum only, hence 

 it is well always to have a supply of this medium on hand ; 

 a few tubes will suffice, as it dries up very rapidly. The 

 blood may be obtained from an abattoir. After the animal 

 begins to bleed a coagulum is soon formed on the hair around 

 the wound. Then the blood is allowed to flow into sterilized 

 flasks, which are well plugged with cotton as soon as filled. 

 The blood-coagulum acts as a mechanical barrier to contam- 

 ination of the blood by the bacteria which always are found 

 on the hair and skin. For this reason no blood is taken until 

 this coagulum has formed. As many flasks may be filled as 

 desired. After a firm coagulum has formed in the flasks they 

 are placed on ice for forty-eight hours. The clear superna- 

 tant fluid is then removed with a sterile pipette and trans- 

 ferred to sterile tubes. Frequent handling is not advisable, 

 as it increases the possibility of infection. 



The method of sterilizing the tubes containing the blood- 

 serum varies. If a liquid medium is wanted, they are exposed 

 to a temperature of 60 C. on each of five consecutive days. 

 If a solid medium is wanted, they are exposed to a tempera- 

 ture just below the boiling-point (100 C.) for one hour on 

 each of two successive days. The medium is solidified in 

 the slanting position. The color of the finished product is 

 usually grayish and opaque. If it contains many red blood- 

 corpuscles, it is of a reddish color. The serum may be ster- 

 ilized either in Koch's blood-serum sterilizer (Fig. 10) or in 

 the ordinary steam-chest ; but the latter should not be closed 

 tightly, because if the temperature is raised too rapidly the 



