104 LABORATORY EXERCISES IN BACTERIOLOGY. 



white mass, like the boiled white of an egg. This step should not be hastened, and the 

 temperature should not be allowed to go above the limit mentioned, lest the surface 

 of the solid serum be blistered by bubbles and rendered unfit for use. After the serum 

 is thus solidified firmly it is transferred to the steam-bath and sterilized in streaming 

 steam by the fractional method. 



A fault in the product of both the old and the present methods of preparation 

 is due to the formation of a dense film on the surface of the serum, from drying, within 

 a comparatively short time. To obviate this it may be recommended that a drop of 

 glycerine should be placed in each tube of the liquid serum before it is subjected to 

 coagulation; or it may be prevented by covering the mouth of the tube with a rubber 

 cap or similar device. 



Exercise 29.- Each student, having previously prepared a pipette 

 (fifty cubic centimeters) by placing a cotton stopper in the upper end 

 and then sterilizing in the autoclave, should withdraw fifty cubic centi- 

 meters of serum from the sedimentation jar, where it has previously been 

 collected from the blood, before the class and distribute it to one dozen 

 culture tubes. These are then arranged in proper slanting position in 

 a wire cage and placed in the dry-air oven, where they are to be heated 

 until coagulated firmly. Heating should be cautiously done, the tem- 

 perature raised slowly to 8o-85 C. and maintained at this point until 

 the coagulation is definitely started, when it is further increased to 90 

 C. for ten minutes longer. It will probably require from the first about 

 half to three-quarters of an hour for completion of the coagulation, and 

 close watch should be maintained upon the thermometer of the oven 

 during the time. Overheating will cause failure from blistering the sur- 

 face of the serum, such tubes being of no further service. After the serum 

 is set the tubes are to be transferred to the steam sterilizer, the top of 

 which is loosely applied, and steamed for twenty minutes on three suc- 

 cessive days. 



Loeffler's Glucose-bouillon-serum. For its particular value in the cultivation of the 

 organism of diphtheria this modification is suggested by Loefner and is of popular use. 

 Three parts of liquid serum are mixed with one part of sterile glucose bouillon (prefer- 

 ably, the latter should be made from veal) . The subsequent steps of preparation are 

 the same as outlined for ordinary serum. 



6. Milk. Aside from its value as an ordinary culture medium, milk is made 

 use of in bacteriologic work in determination of the production of rennet-forming 

 ferments by bacteria, as well as in the study of acid formation. The elements which 

 are of principal nutritive value to the bacteria in milk are its proteids, milk-sugar, and 

 the salines. For such use the milk should be obtained as fresh as possible and 

 should be freed from fat. This last may be done by allowing the cream to separate 

 spontaneously from the milk in a sterile jar in the refrigerator and then siphoning or 

 pipetting the milk from beneath the layer of cream into the culture tubes; or, as is 

 more convenient in most laboratories, fresh separator milk from the dairy may be 

 employed and at once distributed to the tubes. The reaction of the milk should first 



