264 AIDS TO BACTERIOLOGY 



seeding tubes have to be left unplugged. The process 

 is no improvement on the Rideal- Walker test. 



Organic Matter in Disinfectant Testing. Many forms 

 of organic matter have been suggested for incorporation 

 into the bacteriological test. Milk of various strengths, 

 washings from schoolroom floors, and faeces, have been 

 suggested and used; but these either (in the case of milk) 

 never want disinfection in practice, or allow no comparison 

 between two disinfectants, owing to the impossibility 

 of different workers using identical material. 



Sommerville and Walker are undoubtedly right in 

 insisting that the forms of organic matter included in the 

 test should be of a simple form. The false coefficients 

 obtained with oxidising disinfectants can be revealed in a 

 most satisfactory manner by the use of urine, mucin, 

 peptone, casein, gelatin, or blood. Sommerville and 

 Walker first dilute the disinfectant in the proportion 

 recommended by the manufacturers or sanctioned by 

 use, all further dilutions being made with the organic 

 solution. The disinfectant is allowed to remain in contact 

 with the organic solution for one hour before adding the 

 test organism. They prefer as a diluent a solution con- 

 taining 0*5 per cent, gelatin and 0'5 per cent, rice starch, 

 the latter being added to meet the needs of adsorption. 

 A coefficient obtained with this diluent and allowing the 

 preliminary hour's contact of organic matter with the 

 disinfectant is known as a ' Sommerville- Walker co- 

 efficient.' 



In the ' Lister Institute method,' the faeces are dried, 

 first in a water- bath and subsequently at 105 C., ground 

 to a fine powder in an agate mortar, and passed through 

 a fine sieve with a mesh of 130 to the inch. Quantities 

 of 0-15 gramme are added to test-tubes, to which 2 '5 c.c. 

 of distilled water are added, and the tubes are sterilised. 

 Different amounts of a suitable dilution of the disinfectant 

 are added to each tube, together with enough distilled 

 water to make 5 c.c. This gives different concentrations 

 of the disinfectant in presence of 3 per cent, of faeces. 

 The tubes are inoculated in the same way as when the 

 test is made with distilled water. Fifteen minutes is 

 allowed for the disinfectant to act; the test is done at 

 20 C., and an exactly similar experiment is done with 

 phenol. 



