146 G. HAROLD DREW. 



hydrate does not appreciably interfere with the test, though it renders 

 it somewliat less delicate. 



The presence of nitrates or nitrites remaining in the culture fluid 

 after the bacterial action was tested by the extremely delicate Brucine 

 and Diphenylamine reactions. 



THE INVESTIGATION OF SAMPLES OF WATER TAKEN 

 OFF PORT ROYAL, JAMAICA. 



In Jamaica, a measurement of the rate of denitrification in the modified 

 Gran's medium inoculated with samples of sea-water was made, but the 

 isolation of the bacteria on solid media was not attempted, owing to 

 lack of apparatus. 



Samples of sea-water were collected in sterilized stoppered bottles 

 from tlie surface, and from depths of three and six fathoms, in positions 

 about five miles from shore, where, from a consideration of the wind 

 and tide, the water was probably under truly oceanic conditions, and 

 unaffected by the neighbouring land. 



Tlie cultures were kept in a moderately dim light, and the room 

 temperature ranged between 25° and 31'5° C. The average tempera- 

 ture during the growth of each culture was noted. 



In a typical culture made from surface water, and for which the 

 average temperature was 29° C, the first indication of the formation of 

 a nitrite, as given by the Metaphenylene diamine reaction, appeared 

 after twenty-seven hours : after thirty-eight hours the brown colour 

 produced in this reaction was very intense, the culture became cloudy, 

 and on testing with Nessler's reagent, slight Ammonia formation was 

 apparent. After forty-eight hours the culture became very cloudy and 

 a scum of bacterial growth developed : the nitrite and Ammonia re- 

 actions remained unaltered. After sixty-three hours the nitrite 

 reaction was somewhat less marked, the Ammonia reaction was un- 

 altered, and bubbles of gas began to appear. After seventy-two hours 

 many bubbles of gas were being produced and the nitrite and Ammonia 

 reactions were very slight. After eighty-six hours the bubbling had 

 ceased, and no nitrite or Ammonia was present in the cultures. Test- 

 ing the culture for nitrates by the Diphenylamine and Brucine methods, 

 showed that no nitrates or nitrites were left in the solution. In the 

 absence of a gas analysis apparatus the nature of the gas evolved could 

 not be exactly determined, but considering that it was non-inflammable^ 

 did not turn lime-water milky, and that the nitrate originally present 

 had been destroyed, it seems strongly probable that this gas was pure 

 Nitrogen. 



Thus at a temperature of 29° C, 0'5 grammes of Potassium nitrate 



