506 PROFESSOR HEDDLE ON THE RHOMBOHEDRAL CARBONATES. 
was overlooked ;—water (here the mere menstruum of the change), filling the 
vision to the obscuration of all other agents, and of all forces not directly con- 
cerned therewith. 
But if it be once admitted that the interchange is equivalentic chemically 
speaking, that is gravimetrically, this of necessity brings with it that it is equiva- 
lentic volumetrically ; every gravimetric equivalent having its own volumetric 
equivalent. It only remains to find the special volume of these equivalents. 
It has been shown years ago by Orto that this is most simple— We have 
only to divide the equivalent weights by the specific weights, in order to obtain 
a quotient expressing the relations of their volumes.” 
Now the equivalent weight of carbonate of lime is 100; and its specific 
gravity 1s 2°72 ;—3.99 = 368. 
And the equivalent weight of Dolomite is 94; and its specific gravity is 
2°88 ;— 24, = 326. 
When therefore a crystal of carbonate of lime is through the action of 
chemical replacement pseudomorphosed into Dolomite, 100 parts by weight of 
calcite are replaced by 94 parts by weight of dolomite; but this primal 
gravimetric quantity of calcite has a volume of 368, while the replacing quantity 
of Dolomite is only 326 ;—there is therefore, numerically stated, a vacuity of 42 
volumes = 11°41 per cent. or about one-ninth of the original volume. 
Whenever, then, calcite in crystals is replaced by Dolomite, or limestone by 
the percolation of magnesian waters is converted into it, there must be, in the 
case of the first, a central vacuity ; of the second, a more or less general 
porosity.* 
And in lieu of Biscuor’s law, which confessedly failed in the most familiar, 
and out of all comparison the most extended instance, the following presents 
itself :— Whenever, through true chemical interchange, the replacing material 
has an equivalent volume smaller than that of the mineral replaced, the resulting 
solid must be more or less cavernous. 
The general applicability of this law has now to be tested. 
The following table presents in six columns the data as regards the car- 
bonates occurring in nature. 
The first column gives the formule; the second the gravimetric equivalent; 
the third the specific gravity; the fourth the equivalent volume, as obtained by 
dividing the gravimetric equivalent by the specific gravity; the fifth the atomic 
volume, obtained by dividing the last by the number of atoms in the compound; 
and the last gives the angular inclination of the terminal faces of the rhomb,— 
the unequal axis being made vertical ;—showing, as has been before done, that 
* The geologic bearings of so great a shrinkage, or cariousness of a previously solid stratum, being 
the direct consequence of dolomitization, must be apparent. 

