,102 SOLID CULTIVATION MEDIA. 



pass in the least beyond this stage, the transparency is replaced by a 

 dull opacity that becomes denser the longer the serum is exposed to 

 the increased heat. It is necessary, therefore, to remove the test- 

 tubes from the thermostat the moment their contents have become 

 solid, and before the opacity has commenced to appear. If this 

 stage is passed, the solid serum is still a useful nutrient soil, but 

 it has lost that perfect translucency which constitutes its special 

 value, inasmuch as it allows of the exact observation and delineation 

 of the special methods of growth exhibited by special organisms. 

 The amount of heat requisite to inspissate the serum varies in different 

 cases, and chiefly in relation to the various sources from which it is 

 obtained. Thus solidification takes place at a lower temperature 

 in serum from the blood of sheep, oxen, and pigs, and in human 

 pleuritic and peritonitic fluids, than it does in calf-blood serum, or in 

 the fluid from hydrocele. As it becomes solid, a few drops of water 

 are evaporated from the serum and condensed in the tube. In addition 

 to the above-named sources of supply may be named the contents of 

 blisters, and of any of the serous sacs, and the vitreous humour of 

 the calf Before preparing serum from any of the sources mentioned 

 in large quantities, it is advisable to ascertain the amount of albumen 

 present, by heating a sample of the liquid in a test-tube over the 

 flame of a Bunsen burner. If the serum becomes solid, it will 

 answer well for cultivation purposes, and can be prepared as already 

 described. If only a flocculent precipitate is thrown down, it cannot 

 be prepared thus with advantage, as it will refuse to solidify in the 

 thermostat. Such serum, however, can be used in combination with 

 gelatine as a useful culture medium. ^ 



Summary of Process for preparing Sterilised Blood Serum. 



First Day. — Collect blood in sterile jars, and allow to stand for 

 twenty-four hours. 



^ Koch has largely employed, and highly recommends, a sero-gelatint solution, 

 consisting of limpid blood serum, to which five per cent, of its bulk of gelatine is 

 added, and which must be sterilised by the "slow process" above described. It is 

 employed exactly as is the peptonised jelly. Care must be taken that it is not on 

 any occasion heated above 60° C. , when coagulation and opacity would be likely 

 ■ to ensue. (Koch, Mittheilungen a. d. k. Gesundheitsamte, Bd. i. , p. 27. ) 



