514 G. H. DREW. 



The colonies developing in all the cultures were only of two kinds, 

 the Bacterium calcis, and the non-denitrifying species already de- 

 scribed. The non-denitrifying species formed a relatively small pro- 

 portion of the total, and they were not found at all in cultures made 

 from samples taken below 250 fathoms. As they appear to be com- 

 paratively chemically inactive, and as nothing is at present known 

 concerning the part played by them in the metabolism of the sea, they 

 will not be further considered here. 



A consideration of these results obtained in the Tongue of the 

 Ocean shows that the waters down to a depth of somewhere about 

 300 fathoms in April, 1912, contained an enormously larger number 

 of bacteria than the water in the neighbourhood of Tortugas in 

 June, 1911. The number of bacteria falls off from about 14,000 to 

 about 12 per 1 c.c. between depths of 250 and 350 fathoms ; the 

 temperature at 250 fathoms was about 15° C, and at 350 fathoms 

 about 11° C, and it was shown in June, 1911, at Tortugas that 

 B. calcis will grow slowly at 15° C, but that growth is totally in- 

 hibited at 10° C. It would thus seem that the observed distribution 

 of the bacteria agrees fairly with what might be expected from the 

 temperature conditions. 



As regards these observations as to the occurrence of bacteria in 

 small numbers at depths below 350 fathoms, the possibility of ex- 

 perimental error must be considered : a leakage into the water-bottle 

 of 0'25 c.c. as it was being hauled up through the last 300 fathoms 

 would account for the number found, and there are also many possible 

 sources of error in the process of siphoning off the sample, and 

 making the cultures, where a permanent Laboratory is not available. 

 It is possible that the water below 350 fathoms was really sterile, 

 though if so the constancy of the results obtained is curious, if it is to 

 be ascribed to experimental error. In any case, the small number of 

 bacteria found at depths below 350 fathoms can play no part in the 

 metabolism of the sea, since it has been shown that at the temperatures 

 obtaining at these depths B. calcis is incapable of growth. 



The much greater abundance of bacteria in the surface waters of 

 the Tongue of the Ocean than in the waters round Tortugas may 

 perhaps be accounted for by the fact that in the former locality by 

 far the greater part of the surface water must flow over the immense 

 chalky mud flats and shallows which bound it in most directions, and, 

 as will presently be shown, these mud flats are plienomenally rich in 

 bacteria, and are probably still being deposited by bacterial agency. 



