Journal of Applied Microscopy. 237 



Heat each tube in the flame till the liquid boils, in order to expel the carbonic 

 acid and to precipitate the albumen, phosphates, etc. Then drop into each tube 

 a strip of blue and a strip of red litmus paper. These should be immersed in 

 the liquid. Again, heat each tube to boiling and set aside for one minute. Then 

 by means of a glass rod draw out the papers side by side onto the wall of each 

 tube, and when cold compare the colors of the papers by holding the tubes before 

 the window over a white surface. What was originally the blue paper, in tube 

 No. 1, will probably show a red color. Therefore, "2.5 cc. of the deci-normal 

 solution is not sufficient to neutralize the 10 cc. of gelatin. On the other hand, 

 in tube four, both papers may be blue, indicating that 3.5 cc. is more than is 

 necessary to neutralize. The neutral point, that is, where the red and blue 

 papers retain their color side by side, lies therefore between the two extremes. 

 Tube two may still show a slight acid reaction. Tube three may be neutral or 

 nearly so. If the neutral point lies between 3.0 and 3.3 cc, it may be well to 

 measure out fresh portions of 10 cc. of gelatin and to add 3.1 and 3.2 cc. of the 

 alkali. The reaction should be tested as given. In this way it is possible to 

 determine the neutral point to within one-tenth or two-tenths of a cubic centi- 

 meter, and this corresponds to a probable error of only one or two cubic centi- 

 meters of normal alkali per liter. Delicate litmus papers are of course desirable 

 in this process and are better than a litmus solution. 



The above experiment has show^n that 10 cc. of gelatin requires, we will say, 

 3.0 cc. of deci-normal alkali for neutralization. In order to ascertain the amount 

 of alkali necessary for the neutralization of all of the remaining gelatin, this must 

 be measured in a cylindrical graduate. The amount left, we will say, corres- 

 ponds to 950 cc. The amount of deci-normal alkali necessary to neutralize this 

 quantity is ascertained from the following proportion : 10:3:: 950 : x, there- 

 fore X = 285. 



That is to say, in order to neutralize the 950 cc. of gelatin, it would be nec- 

 essary to add 285 cc. of the deci-normal alkali. Since this corresponds to 28.5 cc. 

 of the normal alkali, the latter is added in preference to the large volume of 

 the former, which would unnecessarily dilute the liquid. 



Inasmuch as bacteria grow best on alkaline media, it is advisable to add an 

 amount of alkali over and above that necessary to neutralize the medium. An 

 excess of 10 cc. of normal alkali per liter imparts a desirable alkalinity. Hence 

 'to the 950 cc. of gelatin there will be added 28,5 cc. of normal alkali for neutral- 

 ization, and 9.5 cc. for alkalinity, a total of 38 cc. of normal alkali. 



After the addition of the necessary amount of alkali to the gelatin in the jar, 

 the latter is then placed in the water-bath. The water is now raised to boiling 

 and maintained at this point for three-quarters to one hour. The albuminous 

 substances present in the meat extract now coagulate in flakes and clear up the 

 liquid, so that on subsequent filtration it will be perfectly clear. 



The liquid is then filtered through a plaited filter. This may be obtained 

 ready made, and Schleicher & Schiill's No. 580 is particularly well adapted 

 for filtering gelatin. It is advisable to pass some boiling water through the filter 

 just before filtering the gelatin. If the first portion of the filtrate is cloudy it 

 should be returned to the filter. 



