THERMOSTABILE OR THERMOLABILE SUBSTANCES. 15 



The color of a serum is greatly variable, depending mainly upon its 

 hemoglobin or fat content. Blood taken at the height of the period of diges- 

 tion shows a chylous serum. The serum of nursing women contains milk, 

 that of icteric people contains bile. For most serological examinations 

 these elements in the serum are inert, and do not interfere with the reading 

 of the results. In precipitin reactions, however, the serum must be abso- 

 lutely clear. 



If serum is to be kept for a long time, there are several ways that it may 

 be retained without losing its activity. The method chosen depends upon 

 the serum substance, which is important. 



As will be pointed out again, substances are either thermostabile or 

 thermolabile. The preservation of thermostabile substances (agglutinins, 

 amboceptors) is usually very simple. It is sufficient to place the 

 clear serum which has separated from the clot, into a sterile 

 test-tube plugged with absorbent cotton, and to put it into the 

 ice chest away from the light. To reassure its perfect preserva- 

 tion one may add to it some phenol in such proportion that the 

 carbolic should be present to the extent of 1/2 per cent, solution, 

 e.g., to nine c.c. of serum add i c.c. of a 5 per cent, phenol solu- 

 tion. The latter should be added drop by drop and agitated, 

 so as to avoid the formation of precipitates. Another method, 

 which the author employs almost exclusively for the preserva- 

 tion of amboceptor containing sera, consists simply in inactivating 

 the sera at 56 C. for a half hour and then placing them into the 

 ice chest. Inactivation has the advantage of stopping molecular 

 changes which are produced by ferment actions of fresh serum. 

 Furthermore, heating acts as a sterilizer for isolated air germs 

 which may have found their way into the serum during the 

 process of getting it. In this form, a serum can be kept in the 

 ice box for several weeks without any material change. Occa- F IG g. 

 sionally one finds that a serum will undergo contamination in r Tllbe used 



J f for preserva- 



spite of inactivation, so that it follows, that if a serum is to be pre- tion of 



served for several months, it is advisable to seal it in a test tube, 

 For this purpose a brown glass tube slightly drawn out at its 

 upper end is employed (Fig. 8). The serum is placed into this sterilized 

 tube and then the latter is sealed in the flame at its narrow part. Bacterial 

 and organ extracts are well kept in this way. The best method of 

 preservation consists in evaporating the serum to dryness in a vacuum 

 desicca or. This procedure is rather complicated and can therefore be 

 employed only in institutions. 



A vacuum desiccator with beatable plates is used. The serum is poured in very thin 

 layers into sterile flat dishes and allowed to dry out in the desiccator at a temperature of 

 30 C., later on at 35 C. in a vacuum of 3 cm. mercury. The dried serum forms a 



