On the Precipitation of Calcium Carbonate. 43 



compared to the precipitate of unorganized and amorphous calcium car- 

 bonate. 



Specimens of the precipitates from some of the culture media were sent 

 to Dr. Fred. E. Wright, of the Geophysical Laboratory of the Carnegie 

 Institution of Washington, who described them as follows: 



Preparation I: Precipitate from medium^composed of calcium acetate 5.0 grams, 

 potassium nitrate 0.5 gram, peptone (Witte's) 0.2 gram, sea-water 1,000.0 c.c, filtered 

 and sterilized, contains two substances: (i) Fine grains of a strongly birefracting, appar- 

 ently uniaxial, optically negative substance and with refractive index about 1.66. This is 

 probably calcite. The grains are isolated, and no evidence of spherulitic crystallization 

 was observed. On treatment with very dilute hydrochloric acid a noticeable evolution of 

 carbon dioxide took place. (2) Scattered through the preparation are fine needles of a 

 weakly birefracting substance of refractive index of about 1.525; extinction angle large. 

 These needles are evidently selenite (hydrated calcium sulphate). 



Preparation II: Precipitate from medium composed of calcium succinate 2.0 grams, 

 magnesium tartrate 0.2 gram, potassium nitrate 0.5 gram, sea-water 1,000.0 c.c, consists 

 largely of a cryptocrystalline aggregate of a weakly birefracting substance, whose refractive 

 index is about 1.52 to 1.53. This substance proved too fine for further determination. 

 Scattered through this substance are rounded and irregular patches of a second cr>'pto- 

 crystalline substance of strong birefringence, which gives off CO2 when treated with dilute 

 hydrochloric acid, and is probably calcite. 



Preparation III: Precipitate from a medium composed of calcium acetate 5.0 grams, 

 sodium phosphate (Na2HP04i2H20) 0.25 gram, potassium nitrate 0.5 gram, sea-water 

 1,000.0 c.c, is again very fine-grained and consists (i) in large measure of minute grains of 

 a substance which agrees with calcite in its optical properties in so far as they could be 

 determined. On immersion in dilute HCl a distinct evolution of CO2 gas was observed. 

 (2) Of a substance whose grains are somewhat coarser than the calcite grains, their bire- 

 fringence being medium to weak; refractive index about 1.525; biaxial and apparently 

 optically positive; probably selenite, but not crystallized in the usual manner. 



The small quantity of hydrated calcium sulphate present in these 

 precipitates is undoubtedly derived from that in solution in the sea-water 

 with which the media were made up, but the reason of its precipitation is 

 difficult to explain, since no such precipitation occurred in culture media 

 kept uninoculated under similar conditions as control experiments. It 

 would therefore appear that this deposition of calcium sulphate along with 

 the calcium carbonate must in some indirect way be the result of bacterial 

 action, and it would seem a possible suggestion that the odor of sulphureted 

 hydrogen noticeable in the deeper layers of the mud flats might be due to 

 the reduction of the calcium sulphate to a sulphide, and subsequent decom- 

 position of the sulphide. 



These observations have shown that on the chalky mud fiats of the 

 Great Bahama Bank the B. calcis is found in enormous numbers, and also 

 that this bacterium is capable of precipitating calcium carbonate from 

 fluid media containing soluble calcium salts. It would seem a fair deduction 

 that these mud flats, consisting of fine unorganized particles of calcium 

 carbonate, have been precipitated by the action of the B. calcis on the 

 soluble calcium salts carried into the sea by drainage from the land, where 

 extensive and rapid weathering of the limestone rock is in progress. 



