PRESERVATION OF IMMUNE SERUMS 79 



washed through under gentle pressure until the fluid is clear and neutral 

 to phenolphthalein, when the filter is ready for use. After use, the filter 

 should be boiled in distilled water, scrubbed, and dried in the air. 



Preservation in Fluid Form, by Freezing. Freezing a serum often 

 renders it cloudy or causes a precipitate to be deposited, and interferes 

 with the usefulness of a serum that should be absolutely clear. Freezing 

 is the only practicable method so far devised for the preservation of 

 thermolabile substances, such as complement. A small apparatus, 

 named the "Frigo," has been devised for this purpose by Morgenroth. 

 A satisfactory apparatus may be made by constructing a wooden box 

 with a smaller sheet-metal-covered inner compartment, the space be- 

 tween them being well packed with sawdust. This inner box is then 

 filled with crushed ice, and the whole is covered with a lid lined with 

 several layers of felt. 



Preservation in Powder Form. When serum is poured out in thin 

 layers and dried, it forms yellowish, amorphous masses, that may be 

 collected and ground into a powder, which keeps well and forms an 

 excellent medium for the preservation of many immune serums, espe- 

 cially those of the agglutinating type. Various toxins, such as tetanus 

 toxin and cobra venom, may also be preserved in this form. 



The serum or toxin may be spread out in thin layers on large glass 

 plates, or placed in shallow dishes and dried in the incubator. After a 

 few hours the dried serum, which adheres only slightly to the dish, can 

 be removed with a spatula and placed in a mortar, and ground and 

 stored in sealed tubes. 



The drying process is better carried out in vacuo, and the large serum 

 institutes are provided with these special drying apparatus. A simple 

 form may be prepared after the method of Taeze, as follows: Place a 

 large glass bell-jar with a ground base and a large opening at the top on 

 a polished iron plate. Set this on a large tripod, as this will facilitate 

 heating with a Bunsen burner. The serum is placed within the jar in 

 a shallow dish, and the jar fastened to the iron plate with hot paraffin 

 or wax. The opening at the top is closed with a three-holed rubber 

 stopper: one hole carries a thermometer; a second is connected with a 

 manometer (not absolutely necessary), and the third carries a bent glass 

 tube which is connected, by means of thick-walled rubber tubing, to a 

 suction pump. A low flame is kept burning so as to keep the tempera- 

 ture at about 35 C. The degree of vacuum secured makes little differ- 

 ence, and usually that obtained with an ordinary water-suction pump, 

 allowing for leaks in the tubing, is sufficient, rendering manometric 

 measurements unnecessary. 



