150 G. HAROLD DREW. 



water form, and this would be upheld by the ease with which they are 

 killed by exposure to sunlight. On the other hand, the denitrifying 

 bacteria would appear to be a surface form, reaching their maximum a 

 little below the surface. This would be substantiated by the results 

 obtained in Jamaica, by which it was shown that denitrification 

 occurred much more rapidly in cultures made from samples collected 

 at depths of o and 6 fathoms, and might possibly be explained by the 

 fact that the growth of the bacteria is inhibited by strong light, 



INVESTIGATION OF SAMPLES FEOM A POINT 70 MILES 

 WEST OF USHANT. 



The samples were plated in the Peptone Agar medium after having 

 been kept on ice for twenty-four hours, for the reason previously 

 explained. 



The colonies were well developed after forty-eight hours, and 

 appeared to be all of one kind. A count gave the following average 

 results : — 



Number of colonies developing 

 Depth in fathoms. from 1 c.c. of sample. 



7 



10 

 20 

 30 

 50 

 70 

 80 



9 

 6 

 5 

 6 

 30 

 20 



The increase in the number of colonies at 70 and 80 fathoms is 

 somewhat remarkable, but no conclusions in this respect can be drawn 

 from one series of observations. 



The appearance and cultural characteristics of the bacterium 

 obtained from these samples were identical with those of the denitri- 

 fying form investigated at the Tortugas, with the exception that acid 

 formation did not take place in media containing Cane-sugar. 



1000 c.c. of the modified Gran's medium inoculated on board with 

 10 c.c. of a surface sample immediately after collection, and kept at an 

 average temperature of 20° C, showed the first trace of nitrite forma- 

 tion after 70 hours. After 84 hours a very strong nitrite reaction was 

 obtained, and a slight Ammonia reaction was given with Nessler's 

 reagent. The process of denitrification, even after the lapse of weeks, 

 did not extend beyond this, and no bubbles of gas were formed. Other 

 experiments made with subcultures from Agar and Gelatin media gave 

 similar results, so that it appears that this bacterium cannot entirely 



