272 Development of a Standardised Agar Medmm 



Tests were also made as to the comparative advantages of filtration 

 through filter paper and cotton- wool. No advantage was found in the 

 former method, either with reference to the total number of colonies 

 developing, or to the uniformity between batches of medium separately 



filtered. 



Length and Temperature of Incubation. In working with this medium, 

 the best results have been obtained by incubating the platings at 20° C. 

 for 10 to 12 days. In a shorter period the slow-growing colonies have 

 either not developed or are very small. These results agree with the 

 finding of Cunningham (14). 



5. Tests of the Count Medium. 

 There are two respects in which a medium for use in quantitative 

 work should display uniformity. In the first place, it must be repro- 

 ducible, that is, different batches of medium should be similar in the 

 results obtained with them. Secondly, parallel platings of a suspension 

 of soil, made with a single batch of medium, should develop the same 

 number of colonies within the hmits of random samphng variance. 

 Uniformity in this latter respect will depend mainly upon limitation of 

 the growth of fast growing organisms and especially of moulds and bac- 

 teria that form spreading colonies or develop toxic products, whose 

 chance appearance on platings may affect the number of colonies de- 

 veloping thereon. These two aspects of the medium must be separately 



tested. 



The capacity for colony development on the present medium has 

 been found to be closely reproducible in different batches, if the method 

 of preparation be carefully standardised. In the following test, five 

 batches of medium were separately prepared, and a single suspension of 



