210 Report of the Department of Bacteriology of the 



dextrose has given a higher count than the soil-extract gelatin quite 

 often in the earlier tests but only four times in the last sixteen. As 

 different batches of the media were used in the earlier and later 

 tests, it is quite possible that the variation in the counts may have 

 arisen from this cause alone. Secondly it will be seen that the 

 tap-water gelatin without dextrose has rarely given a count as high 

 as that on soil-extract gelatin; but that all the differences are too 

 slight to show an actual advantage for either formula. Thirdly, it 

 will be noticed that the counts on distilled water gelatin with or 

 without dextrose are still more rarely equal to those on the soil- 

 extract gelatin. In this case also the differences are so slight that 

 their significance is doubtful. These tests cannot be construed as 

 showing any reason for using soil-extract rather than tap-water or 

 even distilled water. If further tests show similar results one of the 

 simpler formulae will unquestionably be considered superior for 

 routine work. 



These tests show that either the gelatin, itself, or the white of 

 egg used in clarification has furnished the bacteria with sufficient 

 nutrient matter to cause large numbers of them to develop into 

 colonies. To determine which of these sources was the more impor- 

 tant a solution of gelatin was made in distilled water and then used 

 without clarification. The counts obtained on it are given in the 

 last column of Table III. Only four tests of this medium were 

 made; but in two of them the count was higher and in one other 

 almost as high as on the clarified soil-extract gelatin. In spite of 

 the small number of tests made, it seems safe to conclude that 

 gelatin is in itself a very satisfactory culture medium for soil-bacteria. 



The use of the soil-extract gelatin was continued, however, 

 throughout the present investigation even though one of the simpler 

 formulae might have given as good results. Its employment in the 

 earlier work made its continued use valuable as a basis of com- 

 parison, and it is plain that the simpler formulas do not give any 

 better results. A further discussion of the merits of this gelatin 

 follows (pp. 219 to 226) in connection with the discussion of the 

 tables in which it is compared with various agar media. 



THE ASPARAGINATE AGAR. 



description of the medium. 

 The asparaginate agar is intended primarily for quantitative 

 work, as it contains no substance of indefinite chemical composition 

 except the agar itself; but it does not allow such great differences 

 in the appearance of the different colonies as does gelatin. The 

 '•omparative tests which follow (pp. 219 to 226) will show whether 

 it meets the other important requirement of a medium for quantita- 

 tive work, that of allowing the growth of the greatest possible number 

 of soil bacteria. 



