THE ACTION OF SOME DENITRIFYING BACTERIA. 147 



were decomposed in eighty-six hours, and it was found that if a further 

 0*5 grammes of Potassium nitrate was added to the culture, it was in 

 turn rapidly decomposed : this could be repeated indefinitely until the 

 other constituents of the medium were used up, or the concentration 

 of waste products became too high. 



Tiie rate of denitrification varied considerably with the temperature, 

 and in cultures kept at a temperature between 10° and 12°C., no growth 

 or denitrification occurred. Denitrification was more rapid in cultures 

 from water taken from a depth of three or six fathoms than from the 

 surface. It was also especially rapid with samples taken from the 

 thick, muddy waters of a mangrove swamp, where organic matter 

 was plentiful. 



The bacteria present in the cultures were very minute, actively 

 motile bacilli with rounded ends. 



THE INVESTIGATION OF SAMPLES OF WATEK TAKEN 

 POUND THE DPY TOETUGAS AND IN THE STPAITS OF 



FLOPIDA. 



Cultures made in the modified Gran's medium from various positions 

 round the Tortugas gave results in close accord with those found in 

 Jamaica, and the average rate of denitrification was the same ; hence it 

 appears probable that much the same bacterial conditions obtain in 

 both places. Cultures were made on various solid media, and pure 

 cultures of the bacteria were isolated. Petri dishes with porous 

 earthenware covers were used and were found of great advantage, as 

 by this means the formation of drops of water on the covers is pre- 

 vented, and the water of condensation evaporates as soon as formed : 

 in addition a free air supply is ensured. If evaporation is progressing 

 too rapidly, the whole Petri dish can be covered with a bell-jar lined 

 with damp filter-paper after the first day. 



By plating samples of surface water from various positions as far 

 ■as possible removed from the influence of the land, an average of four- 

 teen colonies per 1 c.c. sea-water was obtained. These colonies ap- 

 peared to be of two kinds, one much more plentiful than the other. 

 Subcultures made from these colonies in Gran's medium showed that 

 the bacteria forming the most common type of colony produced an 

 active denitrification, while the others grew very slowly in this 

 medium, and produced no denitrification. The chief characteristics of 

 the denitrifying form are as follows : — 



On the Potassium malate, or I'eptone Agar media, colonies are 

 visible as minute white specks after six to eight hours, when the 



