42 PRACTICAL BACTERIOLOGY. 



is obtained, the only really safe method is to employ 

 the greatest care in taking the blood seram, since the 

 blood of healthy animals is free from bacteria, and 

 may be kept free, if all the instruments and apparatus 

 used are thoroughly sterilised, and if the operation is 

 performed quickly and skilfully. For although some 

 bacteria germs are floating in the air, the number of 

 these is small in comparison with those that are cling- 

 ing to the surfaces of surrounding objects, and very 

 few can fall into the nutrient medium if the operator 

 is sufficiently quick in his manipulation. 



The blood serum is solidified at a temperature of 

 from 65° to 68° C. ; this is best done in an incu- 

 bator, the temperature of which can be accurately 

 regulated. The test-tube is not placed upright in the 

 incubator, but in a slanting direction, so that the 

 fluid serum occupies about three-quarters of the length 

 of the tube. After about half an hour to an hour, the 

 serum has become solidified in this slanting position, 

 and then the tube can be taken out and placed up- 

 right. Meanwhile a drop of water has collected at the 

 lower part of the test-tube ; part of it was pressed out 

 of the serum during its solidification, and the rest was 

 condensed on the sides of the tube (condensation 

 water). It serves a double purpose : it prevents the 

 surface of the serum from becoming dry, and also 

 keeps the air in the test-tube saturated with moisture. 

 In order to prevent the evaporation of this water, an 



