154 G. HAROLD DREW. 



DISCUSSION OF EESULTS. 



It seems that the rate of denitrification in equal vohimes of the same 

 culture medium, inoculated with equal volumes of samples of sea-water, 

 must be a function of the number of bacteria in the sample, the tempera- 

 ture at which the cultures are grown, and the specific power of denitrifica- 

 tion of the individual species of bacteria. Considering the rapid multipli- 

 cation of bacteria when the food supply is plentiful, up to a maximum 

 determined chiefly by the accumulation of the waste products of their own 

 metabolism, it appears that the factor of the number of bacteria in 

 the sample may be neglected within the limits of these experiments. 

 For example, the number of bacteria in 1000 c.c. of Gran's medium at 

 the end of twenty-four hours would probably be much the same, whether 

 it were inoculated from a sample containing eight or sixteen bacteria 

 per 1 c.c. Similarly, it was a matter of experience that the first trace 

 of nitrite formation was observable at about the same time, whether 

 5 or 10 c.c. of a given sample had been used for inoculation. 



Consequently it appears that, for purposes of comparison, and with- 

 in the limits of the experiments described, if the temperature be the 

 same for the cultures compared, the rate of denitrification is a measure 

 of the specific denitrifying power of the particular species of bacteria. 



Thus it has been shown that in cultures grown at temperatures 

 approximating that of their normal environment, the denitrifying 

 power of bacteria in seas in the American tropics is considerably 

 greater than that of bacteria in the water to the north of the Bay of 

 Biscay, and that they have the power under favourable conditions 

 of completely breaking down nitrates, which those from the Bay of 

 Biscay do not possess. 



It has also been shown that these bacteria possess the power of 

 precipitating Calcium carbonate from solutions of simple organic 

 Calcium salts, and it is here suggested that some similar action has 

 played an important part in geologic times in the precipitation 

 of various rocks, wholly or in part composed of Calcium carbonate. 



Investigation and comparisons of denitrifying power of samples 

 of the coastal waters of the English Channel have been purposely 

 avoided, as previous experience had shown that the bacterial conditions 

 in this region are so very complicated. It should, however, be men- 

 tioned that in this coastal water denitrifying bacteria are present 

 which are capable of entirely decomposing nitrates into free Nitrogen, 

 e.g. the Bacillus rcjwjis (Gran), but that this complete denitrification 

 is only possible at a temperature far higher than that of their normal 

 environment, and that at a temperature of 15° C. even the forma- 



