38 PRACTICAL BACTERIOLOGY. 



in the following way : — It is prepared from the blood 

 of animals (oxen or sheep) at the slaughter-house. 

 First the skin, and the instruments to be used for the 

 opening of the blood vessels, are freed from germs, or 

 sterilised, in a manner to be described later. An 

 artery is then opened and the blood is received directly 

 into sterilised vessels, which are covered up and 

 allowed to stand in as cool a place as possible until 

 the red blood corpuscles have separated themselves 

 from the fluid plasma; ,this plasma is then drawn off 

 with a pipette, which has been sterilised beforehand 

 by being strongly heated. 



It is best to put the fluid blood seram straight into 

 the vessels which are to be used for the cultivations, 

 that is to say for the most part into test-tubes. These, 

 however, must be most carefully sterilised beforehand, 

 by keeping them for two or three hours at a tempera- 

 ture of 160° C. This is done in the following manner: 

 — The test-tubes are carefully cleansed, and are fitted 

 with tight-fitting plugs of cotton wool. They are 

 then placed upright in a wire basket, which is kept 

 for some hours in the hot-air steriliser at a tempera- 

 ture of from 140° to 160° C. 



The hot-air steriliser (Fig. 3) is a box of sheet-iron, 

 the walls of which are covered with asbestos, in order as 

 much as possible to prevent the loss of heat. It is 

 best for the box to have double walls, between which 

 there is a non-conducting stratum of air. On. the top 



