On the Precipitation of Calcium Carbonate. 13 



followed. Both Baur (i) and Gran (9) made use of calcium salts in their 

 culture solutions in order to obviate the great increase in alkalinity that 

 resulted if potassium or sodium salts were used, but they have not called 

 attention to, or apparently realized, the probable significance of this pre- 

 cipitation of calcium carbonate by bacterial agency as an important factor 

 in the formation of various sedimentary calcareous rocks in tropical seas. 



The subject of the precipitation of calcium carbonate in sea-water has 

 been dealt with by Murray and Irvine (14) in 1889 and again by Murray 

 (Sir John) and Hjort (13) in 191 2, and they ascribe the precipitation to the 

 interaction of ammonium carbonate, derived as an ultimate product of the 

 decomposition of nitrogenous organic matter, with the calcium sulphate 

 present in sea-water, according to the equation 



(NH4)2C03 + CaSOi = CaCOa + (NH4)2S04 



Expressed in the terms of the ionic hypothesis, this reaction can be explained 

 by the statement that CaCOa must be precipitated when the product of the 

 concentration of its ions Ca and CO3 exceeds a certain limit ; an increase in 

 the concentration of CO3 ions is produced by the advent of (NH4)2C03, 

 which is partially ionized into NH4 and COs, and hence the product of the 

 concentrations of Ca and CO3 ions is increased and CaCOs is thrown out 

 of solution. 



Though this reaction has been conclusively shown to occur under experi- 

 mental conditions, where nitrogenous organic matter has been allowed to 

 putrify for some time in sea-water, yet it is obvious that its effect must be 

 purely local and must be confined to the immediate neighborhood of the 

 decaying organic body which gives rise to the formation of (NH4)2C03. 



In this paper the precipitation of CaCOa in an unorganized state alone 

 is dealt with. The formation of the calcareous skeletons, tests, and shells 

 of animals, and the skeletons and platelets of algae, which play an immensely 

 important part in the constitution of marine bottom-deposits, is beyond the 

 scope of this work. 



DESCRIPTION OF APPARATUS. 



In 191 1 the apparatus at my disposal was of a somewhat primitive 

 nature, as it is difficult to know beforehand exactly what gear will be nec- 

 essary in a new field of work. In 1912 a more complete outfit was available, 

 and the Carnegie Institution's yacht Anton Dohrn was especially fitted for 

 my requirements. 



For deep-sea work the motor trawl winch was modified so as to carry 

 fine sounding-wire, and a derrick was rigged aft, projecting over the stern 

 of the boat, over which the wire was led. The motor winch is sunk below 

 the level of the deck, a commendable arrangement, as it can be covered 

 over with hatches when not in use and so affords great economy of deck 

 space, and also has the advantage of bringing the weight of the winch nearer 

 the water-line and avoiding the unstability that may be caused when a 

 heavy winch is fixed on deck. 



