48 



BACTEEIOLOGY 



ascertained whether the serum is sterile, which is most 

 readily done by covering the test-tubes with india-rubber 

 caps and letting them stand for a few days in the incubator. 

 Only those test-tubes should be used in which no germs of 

 micro-organisms have visibly developed. 



The serum of human blood is often employed, and can 

 be obtained sometimes at operations and sometimes from 

 placentae. 



Fig. 23. — Apparatus for tke Inspissation op Blood Sekum. 



Modifications of serum. — Those fluids which are pro- 

 cured from hydroceles, ovarian cysts, or dropsical effusions, 

 are very nearly akin to human blood-serum, and the 

 process for manufacturing nutrient media from them is 

 similar. 



Loffler modified the blood-serum as follows : — Having 

 freed an aqueous extract of meat from albuminoid bodies, 

 he added 1 per cent, of peptone, 1 per cent, of grape-sugar, 

 and 0-5 per cent, of common salt to it. This solution, 

 which has an acid reaction, is neutralised with sodium 

 bicarbonate, then sterilised in the steam apparatus, and, 

 after cooling, mixed with liquid blood serum, in the propor- 



