GELATIN AND POTATO MEDIA. 157 



added in preference to the large volume of ft alkali, which 

 would unnecessarily dilute the liquid. 



Bacteria grow best on slightly alkaline media. It is, 

 therefore, desirable to add an amount of alkali, over and 

 above that necessary for mere neutralization. An excess 

 of 10 c.c. of N NaOH per liter imparts a desirable alkalin- 

 ity. Hence, to the 950 c.c. of gelatin there will be added 

 28.5 c.c. of M NaOH for neutralization, and 9.5 c.c. for alka- 

 linity—a total of 38 c.c. of N NaOH. 



After the addition of the necessary amount of alkali to 

 the remaining gelatin in the jar, the latter is then placed in 

 the water-bath. The water is now raised to boiling, and is 

 maintained at this point for %-l hour. The albuminous 

 substances, which are present in the meat [extract, . co- 

 agulate in flakes, and clarify the liquid so that on subse- 

 quent filtration it will be perfectly clear. 



The liquid is then filtered through a plaited filter. 

 These may be obtained ready made. Schleicher and 

 Schiill's No. 580 is particularly well adapted for filtering 

 gelatin. It is advisable to pass about 200 c.c. of boiling 

 water through the filter, before beginning the filtration. If 

 the first portion of the filtrate is cloudy, it should be 

 returned to the filter. The filtrate should be (1) perfectly 

 clear; (2) should be neutral or slightly alkaline in reaction; 

 (3) should not become cloudy or coagulate when boiled in a 

 test-tube for 1 to 2 minutes; (4) should solidify when cooled. 



If the filtrate is cloudy and strongly alkaline, correct 

 the reaction by the addition of a few drops of dilute acetic 

 acid. If it becomes cloudy, or coagulates when warmed, 

 continue heating in the water-bath, A cloudiness can be 

 cleared up by the addition of a white of an egg dissolved 

 in a little water. The commercial, dry egg-albumin is con- 

 venient for this purpose. In either case, the albumin solu- 

 tion should not be added to the boiling solution, but this 

 should first be cooled to about 60°. The temperature should 



