Aragonite is said to be about 10 per cent more soluble than calcite, but no difference in the point of equilibrium of the two substances with sea-water was detected in these experiments (fig. 8). This may be explained by the fact that a few calcite crystals were mixed with the aragonite, equilibrium was only approximated, and there were slight errors in the determinations.
During the rough weather, white calcareous mud is stirred with the sea-water at Key West and to a lesser extent at Tortugas, and it was thought possible that the mud granules might form nuclei for precipi tation and explain the low alkaline reserve at Tortugas and lower alkaline reserve at Key West. On agitating white calcareous mud, dredged from the bottom, with sea-water, no definite equilibrium was reached, even at the end of 4 days. If the alkaline reserve was first lowered by removal of some it remained lower than if shaken with calcite, and if normal sea-water was used the alkaline reserve remained higher than with calcite. It was thought possible that the grains were covered with an impenetrable film of organic matter; so some mud was dried and powdered in a mortar to break the pieces and form fresh surfaces, but similar results were obtained with this.
If mud was mixed with an equal weight of calcite, the results were the same as with pure calcite. This mud and calcite on standing in sea-water for 30 days had not all changed to calcite. All these facts tend to show that the particles in the mud are in some way retarded or prevented from getting into equilibrium with the water.
In order to be sure of clean surfaces of natural calcareous substances, a specimen of coral, Mceandra clivosa, was ground and powdered in a mortar and agitated with sea-water in the same manner as in previous experiments. The results were similar to those with mud (fig. 8).
There seems to be a more soluble form of calcium carbonate (the of Johnston), but since it can not be obtained in a pure state no attempt was made to prepare it. One experiment, however, was made with a precipitate of that appeared as spherical grains under the microscope. It was agitated for 12 hours with sea-water and the alkaline reserve was 0.0022 at pH 7.95. No further experiments
were made to determine whether equilibrium had been approximated.
These experiments clearly show that the surface water of the sea is a supersaturated solution of and it is only necessary to intro duce calcite crystals in order to cause considerable precipitation of this substance. Precipitation goes on in the bodies of organisms in the surface waters of all seas. The precipitation observed by Vaughan at Tortugas is very finely divided, but whether it was formed in the bodies of minute organisms, which subsequently died, has not been determined. Such particles might slowly grow, since the agitation of them with sea-water was found to take a trace of out of the water. Small crystals have been seen in the bodies of Protista and, whether they are or not, they might form nuclei for the precipi tation of if released into the sea-water.
In some experiments in liter flasks of resistance glass, filled to the neck (and hence admitting but slight loss of the pH and alkaline reserve were determined immediately before and after agitation with calcite. The loss of from the sea-water was estimated in two ways: (1) the remaining was calculated from the pH and alkaline reserve; (2) the loss of was calculated from change in alkaline reserve.
In table 25, the agreement is very striking, in view of the probable error in determination of pH and the liability to loss of from the water surface in the neck of the flask, agitated by the rotary stirrer. If the pH of sea-water should be maintained (by the action of plants) at 8.2 while it was agitated with calcite crystals, the loss of would be about 0.001 N, or 0.0005 M, or 0.1 gram per liter. This would cause a deposit of 10 kg. per square meter of bottom in water 100 meters deep. This would cause a lowering of the total calcium content of Tortugas sea-water by about 4.5 per cent.
The actual precipitation of was most noticeable in the Mar quesas lagoon. At 4 p. m., July 30, the pH was 8.46 and there was a precipitate of coming down in the water and incrusting the eel-grass.