620 APPLICATION OF METHODS OF BACTERIOLOGY 



Another point of equal importance in its influence upon 

 the number of colonies that develop is the reaction of the 

 gelatin. A marked excess of either alkalinity or acidity 

 always has a retarding efi^ect upon many species found in 

 water. Fuller's experience at the Lawrence (Mass.) Ex- 

 periment Station has shown that gelatin of such a degree 

 of acidity as to require the further addition of from 15 to 

 20 c.c. per litre of a normal caustic alkali solution to bring 

 it to the phenolphthalein neutral point gives, on the whole, 

 the best results. Thus, by way of illustration. Fuller found 

 that a sample of Merrimac River water gave 5800 colonies 

 per c.c. on phenolphthalein neutral gelatin, 15,000 colonies 

 on gelatin that would need 20 c.c. of normal alkali solution 

 to bring it up to the phenolphthalein neutral point — i. e., 

 a feebly acid nutrient gelatin, and 500 colonies on a gelatin 

 so alkaline as to require 20 c.c. of a normal acid solution 

 to bring it back to the phenolphthalein neutral point. 



Throughout this part of the work it is to be borne in 

 mind that when reference is made to plates it is not to a 

 set, as in isolation experiments, but to a single plate. 



Method of Counting the Colonies on Plates.^ — For conven- 

 ience in counting colonies on plates or in tubes it is customary 

 to divide the whole area of the gelatin occupied by colonies 

 into smaller areas, and either count all the colonies in each 

 of these areas and add the several sums together for the 

 total, or to count the number of colonies in each of several 

 areas, ten or twelve, take the mean of the results and multiply 

 this by the number of areas containing colonies. The latter 

 procedure obtains, of course, only when all the areas are 

 of the same size. By this method, however, the results 

 vary so much in difl^erent counts of the same plate that they 

 cannot be considered as more than rough approximations. 



