ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 457 



anil replaced on the fire. It is then gradually heated up to boiling, 

 when it is again removed, having been kept the. while constantly stirred 

 with a glass rod. The albumen is separated from the gelatin by 

 filtering while warm through fine cotton cloth and plugging the stem 

 of the filter with cotton wool. Filtration is efi"ected in a few minutes, 

 and the liquid thus obtained is perfectly transparent. 



(4) Nutritive agar:— The clarification of agar may be effected in 

 the same way, but requires, for its dissolution, to be kept boiling for 

 quite a long time. When quite dissolved it is necessary to add some 

 tartaric acid in solution in order to render it acid. When clarified by 

 means of white of egg, it is neutralized with the soda solution. When 

 coagulated the solution becomes opalescent ; but the author states that 

 he hopes shortly to be able to produce agar media as clear as those 

 of gelatin. 



Method of Preparing Nutritive Gelatin.* — Mr. N. A. Moore first 

 sterilizes the tubes to be used by heating them for one hour in a hot air 

 sterilizer or oven at 150° C. Then take, say, 250 grams (about half a 

 pound) of beef from which all fat has been removed. Chop or grind this 

 to a fine pulpy mass. Transfer it to a beaker, and add 500 ccm. distilled 

 water, i. e. 2 ccm. to each gram. Thoroughly stir up and place in ice- 

 box till next day. The meat infusion should then be thoroughly stirred, 

 and the liquid portion separated by filtering and squeezing through a 

 linen cloth. The red liquid thus obtained must be brought up to 

 600 ccm, by adding distilled water. To this is now added 1 per cent, of 

 pepton, 1/2 per cent, sodium chloride, and 10 per cent, of the best 

 gelatin (5 grams pepton, 25 salt, and 50 gelatin). The beaker 

 containing the mixture is now placed in a water-bath, and heated to 

 45° C, and allowed to stand until the gelatin is completely dissolved. 



The next step is to add, drop by drop, a nearly saturated solution of 

 sodium carbonate to the beef-infusion-pepton-gelatin mass until the 

 reaction is slightly alkaline. (If it be made too alkaline this condition 

 may be neutralized by acetic acid.) It is next clarified by adding the 

 whites of two eggs, and the mixture is then boiled for half an hour in a 

 water-bath. It is next allowed to cool and set, and then reboiled and 

 filtered in a hot-air filter at 60° C. into the sterilized tubes (7-8 cm. in 

 each). If not perfectly clear it must be refiltered. After they have 

 been filled, the tubes are sterilized in a steamer at 100° C, or three 

 successive days for 10 minutes, or they may be boiled for 5 minutes in 

 a water-bath. If the gelatin is boiled too much it will not set on cooling. 



Presence of Nitric Acid in Nutrient Gelatin.f — Dr. R. J. Petri has 

 found that gelatin constantly gives the nitric acid reaction which was 

 first obtained by means of the diphenylamin sulphuric acid reaction, and 

 also by means of the brucin reaction and sulphate of iron with sulphuric 

 acid. As the gelatins used did not give Griess's reaction for nitrous 

 acid, it followed that this compound was a product of bacterial growth. 



The next step was to test the various ingredients of the nutrient 

 gelatins. In meat infusion neither nitrates nor nitrites were present. 

 Peptons examined in the same were almost always found to be free, 

 although in a few preparations, traces of nitrate were discovered. 



^ Amer. Mon. Micr. Journ., x. (1889) pp. 41-2. 



t Ceutralbl. f. Bakteriol. u. Parasitenk., v. (1889) pp. 457-60. 



