GERMICIDAL POWER OF CHEMICAL DISINFECTANTS 1 99 



It will be seen that by preparing the dilutions of the disinfectant according 

 to the strengths shown in the second column of Table 18, and then adding i cc. 

 of organic matter to 4 cc. of the dilutions, the final dilutions of the disinfectant 

 become as represented in the first column of the table given above. 



PREPARATION AND USE OF THE PEPTONE -GELATIN ORGANIC MATTER 



As already stated, we prefer to use a mixture of peptone and gelatin dissolved 

 in distilled water. It is prepared from Witte's petpone (siccum) and "Best 

 French Gold Label" gelatin. The stock preparation is made to contain 10 per 

 cent, peptone and 5 per cent, gelatin. Proportionate amounts of peptone and 

 gelatin respectively, are weighed out and liquefied separately in small quantities 

 of water by means of heat. They are then mixed in a graduate and sufficient 

 water added thereto to make a mixture containing 10 per cent, of peptone and 5 

 per cent, of gelatin. It is then placed in bottles of appropriate size and sterilized 

 on 3 successive days. When the mixture has become cold it will be observed 

 that some of the peptone settles to the bottom of the bottles as a flocculent 

 deposit. Consequently, the bottle should be shaken before using it. The 

 stock preparation containing 5 per cent, gelatin becomes semi-solid if it is kept 

 in the cold room at a temperature of i6C. However, by warming it until it be- 

 comes perfectly liquefied and then not allowing it to go below the temperature of 

 2OC. we found that it remains liquid and is easily measured in the pipette. 



By adding i cc. of the stock preparation of 10 per cent, peptone and 5 per 

 cent, gelatin to 4 cc. of the disinfectant dilutions a resulting mixture is obtained 

 containing 2 per cent, of peptone and i per cent, of gelatin, or a total of 3 per 

 cent, of organic matter. 



THE METHOD OF CONDUCTING THE TEST 



Four cc. of each disinfectant dilution, including the phenol controls, are ac- 

 curately measured into the seeding tubes and placed in the water bath. For 

 reasons that will be obvious later, it is difficult to handle more than nine dilu- 

 tions at one time in making a test. The broth culture of B. typhosus is filtered 

 and placed in the water bath. The desired quantity of stock peptone-gelatin 

 mixture is measured into a large test-tube or flask and also placed in the water 

 bath. It is necessary to have a slight excess of the organic matter, so that there 

 will be no trouble in getting the lo-cc. pipette full when adding the organic 

 matter and culture to the disinfectant. Thus, for nine tubes we usually meas- 

 ure out 15 cc. of the organic matter When the disinfectant dilutions, the ty- 

 phoid culture, and the organic matter have reached the temperature of the water 

 bath (2OC.) 1.5 cc. of the typhoid culture is added to the 15 cc. of organic 

 matter and thoroughly mixed by means of a lo-cc. pipette. With the same 

 pipette the seeding tubes containing the disinfectant dilutions then have added 

 to them, successively every 15 seconds, i.i cc. of the mixture of the organic 

 matter and typhoid culture. The technique of shaking, planting, etc., is then 

 as has already been described. 



