On the Precipitation of Calcium Carbonate. 25 



greatest elevations are the hurricane ridges, which are not more than 15 

 feet above sea-level, and during a hurricane the islands are sometimes 

 completely submerged. There is no vegetation on the smaller keys, but 

 Loggerhead Key, on which the Marine Biological Laboratory of the Car- 

 negie Institution of Washington is situated, is partially covered with a 

 growth of bushes and coarse grass. There is no fresh-water supply on the 

 islands. 



From these considerations it is obvious that the risk of contamination of 

 samples of sea-water taken a few miles from the Keys through land bacteria 

 is very small, and that such samples may be taken as being truly oceanic. 



The motor yacht Anton Dohrn and smaller motor boats made the col- 

 lection of samples an easy matter, and the well-equipped laboratory made 

 possible fuller investigations than those attempted in Jamaica. 



A number of cultures were made in Gran's medium under conditions 

 exactly comparable to those made at Port Royal, and the rate at which the 

 process of denitrification proceeded was observed. The results agreed 

 almost exactly with those obtained at Port Royal, so need not be described 

 in detail. It thus seems that the denitrifying power of the bacteria in the 

 seas around the Tortugas is the same as that of those around Jamaica. 



Cultures were also made on various solid media, and pure cultures of 

 the various species of bacteria were isolated by plating in Petri dishes 

 with peptone agar. Samples of surface water taken from various positions 

 around Tortugas as far as possible removed from influence of the land, and 

 collected on sunny days, gave an average count of 14 colonies per i c.c. 

 of sample. Counts of several plates from the same locality, and from 

 different localities, showed a somewhat remarkable agreement as to the 

 number of colonies present, the highest count ever obtained being 20 and 

 the lowest 8 per i c.c. Allowing for experimental error, this shows great 

 uniformity in the distribution of bacteria in the sea around Tortugas. 



The colonies appeared to be of two kinds when grown on peptone agar, 

 one much more plentiful than the other. Subcultures made from these 

 colonies in Gran's medium showed that the bacteria forming the most com- 

 mon type of colony produced an active denitrification, while the others 

 grew very slowly in this medium and produced no denitrification. 



The characteristics of the denitrifying form are as follows : 



On the potassium malate, or peptone agar media, colonies are visible as 

 minute white specks after 6 to 8 hours when the room temperature averages 

 29.5° C. After about 18 hours the colonies are well developed; they are 

 white in color, circular but with finely irregular outline, and have a granular 

 appearance. Superficial colonies are much elevated at first, but as growth 

 proceeds they spread rapidly over the surface of the agar. Deep colonies 

 remain small, circular, and discrete. 



Growth is somewhat more rapid on peptone agar than on the potassium 

 malate agar, and the older colonies develop a brownish tinge in the center 

 when growing on the former medium. On gelatin peptone (5 per cent 



