The Several Forms of Calcium Carbonate. 493 



a = 154, 7 = l - 63, if formed under benzol; after two days 

 these crystals had changed almost entirely to well-formed 

 rhombs of calcite. In marked contrast to the rapid disappear- 

 ance of this hydrate when in masses* is the preservation for 

 months of separate well-formed crystals in clove oil under a 

 microscope cover-glass ; this is possibly analogous to the well- 

 known fact that a perfect crystal of Na,CO 3 .10H 2 O may be 

 kept for a considerable time in the air, whereas imperfect crys- 

 tals will rapidly effloresce. The hexahydrate changes rapidly 

 at room temperature into calcite and water, and this process 

 goes on in a few daysf even at 0° showing thus that the hexa- 

 hydrate also is unstable with respect to calcite at all tempera- 

 tures above 0°. On the other hand, it is claimed:}: that crystals 

 of hydrated carbonate have been found in wells and pumps. 

 Accordingly this hydrate (or other hydrate) may conceivably 

 be precipitated under natural conditions : viz., if it happen that 

 water — in particular sea water — at a temperature of nearly 0° 

 should become supersaturated with respect to CaC0 3 ; more- 

 over if it does occur, it might readily pass undetected, for, by 

 reason of its instability, examination of such material, if not 

 undertaken immediately, would fail to disclose anything but 

 calcite. 



Before passing on to the consideration of the relations 

 between the several forms, we give in Table I a summary state- 

 ment of that portion of our experimental preparative work 

 which we believe to be significant in relation to the previous 

 discussion of the conditions of formation of the several forms 

 of calcium carbonate. Most of these experiments were carried 

 out in the way already described, but in a few cases (brought 

 together in Table I b) special methods were employed ; many 

 of the experiments cited were repeated several times. 



* This may be attributed to the presence in the crystal masses of intersti- 

 tial water which, by its solvent action, would hasten the transformation. 



f Cf. F. Vetter, Z. Kryst., xlviii, 72, 1911. Eeally good crystals, however, 

 persist for some weeks in contact with water at 0°. 



% Pfeiffer (Arch. Pharm. [2], xv, 212) who considered them to be pentahy- 

 drate (cited from Thorpe's "Dictionary of Applied Chemistry," article on 

 calcium carbonate). 



