80 CLINICAL BACTERIOLOGY. 



cent, solution of grape-sugar, of which 20 cu. cm. are 

 added to each liter just before introduction into the 

 test-tubes, in order to limit as far as possible the caramel- 

 ization of the grape-sugar that occurs on protracted heat- 

 ing. Grape-sugar bouillon is best sterilized by exposure 

 for ten minutes in the steam-chest on three successive days. 

 In order to determine whether a given bacterium generates 

 acid, a few drops of sterile tincture of litmus may be added 

 to the bouillon. 



(&) Preparation of Gelatin. The same steps are fol- 

 lowed as in the preparation of bouillon, except that to 

 every liter of meat-infusion 100 grams (ten per cent.) of 

 gelatin are added besides ; then follow solution by boiling 

 in the steam-chamber, alkalinization, addition of the white 

 of an egg, boiling for an hour, and filtering. In the case 

 of gelatin also the reaction must be tested after boiling. 

 Gelatin thus prepared remains firm up to temperatures of 

 about 24 C. (75.2 F.). If gelatin is desired that remains 

 firm at still higher temperatures (27 C. 80.6 F., 28 

 C. 82.4 F.), the procedure is modified by reducing the 

 heat applied to the lowest possible minimum. Forster has 

 determined that the solidification-point of gelatin is reduced 

 about 2 for every hour's heating. The most practicable 

 method of preparing gelatin is, therefore, as follows : To one 

 liter of Loftier' s nutrient bouillon heated in a vessel over a 

 small flame to about 60 C. (140 F.)are added 100 grams 

 of commercial gelatin cut in strips. With constant stirring 

 complete solution of the gelatin is effected by boiling for 

 seven minutes. Then the fluid, rendered acid by the pres- 

 ence of the gelatin, is carefully alkalinized, and, after addi- 

 tion of the white of an egg, is boiled for fifteen minutes in 

 a Papin dish ; it is next filtered over a water-bath at a tem- 

 perature of 60 C. (140 F.) into a large flask, and is then 

 distributed among test-tubes (ten cu. cm. to each). The 

 test-tubes are next sterilized by exposure for fifteen minutes 

 in a Papin dish. It is advisable to employ a perforated tin 

 shelf, in the openings in which the tubes may be placed in- 

 dividually, so that each is entirely surrounded by water. 

 To expedite the process of heating, this shelf is at first 

 rotated in the steam-cylinder. The nutrient gelatin thus 

 obtained is completely clear, remains firm at a temperature 

 of 27 C. (80.6 F.) or 28 C. (82.4 F.), and is always 

 sterile. It may be permitted to harden in vertical columns 



