44 PRACTICAL BACTERIOLOGY. 



operator, and these germs would develop. In order 

 to get rid of thenij the cotton-wool, since they are 

 only on the surface, may be singed in a flame or 

 dabbed with a few drops of corrosive sublimate solu- 

 tion (1 : 1000). An air-tight, closely- fitting india- 

 rubber cap, which has been lying for some hours in 

 corrosive sublimate solution, is then immediately drawn 

 over the cotton-wool stopper, without having been 

 previously dried from the corrosive sublimate. 



Blood serum, prepared in this way, remains un- 

 altered for a long time, except that it gradually loses 

 its nourishing properties ; the fresher it is, the more 

 suitable is it for cultivations ; it can, however, be kept 

 for six or eight weeks without serious loss of nutrient 

 power. Some of the tubes, despite the greatest care, 

 are sure to contain living bacterium and mould germs, 

 and are, in consequence, useless. After a few days 

 the colonies of bacteria and of mould fungi make their 

 appearance, and then these imperfectly sterilised tubes 

 can be separated from the others. A peculiar fatty 

 film of cholesterin nearly always appears on blood 

 serum ; this must not be confused with colonies of 

 bacteria. 



The gelatine, prepared in the manner described 

 above, is in the same way poured off into test-tubes, 

 till they are about one-third full. Especial care must 

 be taken that in filling the tube no gelatine should 

 run down on the inside where it is touched by the 



