72 BACTERIOLOGICAL METHODS 



should respond to the following test: Place 0.30 gram of the 

 gelatin in a medium sized test-tube and add 15 cc. of distilled 

 water, let stand for half an hour, warm gently until all of the 

 gelatin has dissolved, then place the tube in water at a tempera- 

 ture of 15.5 C. and leave undisturbed for half an hour. The 

 solution should remain in place when the tube is inverted. 



The commercial gelatin is a variable product, being made from 

 varying proportions of animal tissues as hides, ligaments, bone and 

 bone cartilage. The purest and best gelatin is made from liga- 

 ments and this kind would no doubt give the most uniform re- 

 sults in bacteriological work, but it is apparently not possible 

 to obtain such gelatin in the market. The next best grade (prac- 

 tically obtainable) would be that made from hides of compara- 

 tively young domestic cows free from all foreign additions as salt, 

 arsenic and other hide preservatives. 



Each lot of gelatin should be examined microscopically before 

 making it into culture media. Old yellowed and brittle material 

 should not be used. Examine from five to six sheets from each 

 pound package, using the low power of the compound microscope. 

 The examinations are made directly without mounting. If numer- 

 ous mold colonies are found as shown in Fig. 19, or numerous 

 mold filaments more or less scattered through the mass, it is 

 unfit for use as a culture medium. Numerous formed mold 

 colonies in the matrix indicate growth during the drying process 

 after the gelatin was spread on the drying screens. More or 

 less torn and disintegrated hyphal fragments unequally distributed 

 through the mass indicate infection and growth before the 

 gelatin was spread for drying. To examine for bacteria, mount 

 small bits of the sheet on a slide in water covered with cover 

 glass. If bacteria are numerous, approximating 10,000,000 per 

 cc. and more, it should not be used. In order to make more 

 accurate counts, take i gram of the gelatin and rub up in 9 cc. 

 of boiled distilled water and make the counts of the thoroughly 

 mixed sample by means of the hemacytometer. As a rule it is 



