166 CULTURE MEDIA 



is too great for the growth of many organisms. The acidity 

 is sufficiently corrected by soaking the pieces of potato in a 

 1 per cent, solution of sodium carbonate for an hour before 

 they are put into the tubes or plates. 



Glycerinized potato tubes are conveniently prepared by 

 covering the potato in the tube with glycerin broth, steril- 

 izing and pouring off the excess broth immediately after 

 sterilizing, taking care that the tubes do not become con- 

 taminated which is not very probable if the work is quickly 

 done while the tubes are still hot. 



Blood Serum Media. — Blood serum, usually from the largfer, 

 domestic animals on account of convenience in securing it in 

 quantity, is used in the study of the bacteria causing disease 

 in man and animals. Most commonly the serum is collected 

 from the clotted blood after it has well separated (usually 

 about forty-eight hours is required for this). It is then run 

 into tubes which are plugged with cotton and placed in an 

 an apparatus for coagulating the serum by heat. A copper 

 water bath with a tightly closed air compartment or the hori- 

 zontal autoclave (Fig. 80) is sufficient for this purpose, though 

 special forms of apparatus are to be had. It is important that 

 the temperature be raised slowly so that the blood gases escape 

 gradually. Three to five hours or longer should be allowed for 

 the temperature to reach the boiling-point. If the tubes are 

 heated too rapidly, the serum is filled with bubbles and badly 

 torn since the gases are driven off suddenly. Loffler's serum is 

 made by adding one part of dextrose broth to thi-ee parts 

 of serum and then coagulating as above. The solid- 

 ified serum in either case is best sterilized discontinuously, 

 though with care the autoclave at 15 pounds' pressure 

 may be used for a single sterilization. This is very apt to 

 cause a greater darkening of the serum and frequently also 

 a laceration of the solid mass by escaping gases. 



Blood serum is also used in the liquid state. For this 

 purpose it is best to collect it aseptically; or it may be 

 sterilized discontinuously at a temperature of 55° or 56° on 

 seven to ten consecutive days. Novy has recently suggested 

 dialyzing the serum to free it from salts and thus prevent 

 its coagulation when heated. Whether the removal of the 



