BY MOLECULAR COALESCENCE. 103 
this solution, and the other conditions necessary to form 
a globular compound of the absent substance. And this 
compound, too, actually exists on the side of the cavities 
containing the fluid in which these chemical changes are 
taking place, showing that this compound is not only 
furnished in part directly from the water, but also that 
the principle of its formation is analogous to that of the 
artificial process for obtaiming the same form of carbon- 
ate of hme. Hence, weighing all these circumstances and 
deductions together, it will appear next to certain that 
both inferences are correct, and that tlrerefore the fluid 
contained in these spaces cannot have been deprived of its 
salt of lime before it arrives there. The thinnest septa 
between these cavities consist of several layers of an ex- 
quisitely fine-and perfectly homogeneous membrane, coated 
more or less thickly with calcareous matter. If the layer 
bound immediately the cavity, the surface of it in contact 
with the fluid is, as was before observed, studded with 
particles of carbonate of a globular form; if the layer is 
not so situated, but connected with another layer, the 
calcareous matter is in thin scales, which can be demon- 
strated to result from the flattening out of the globules. 
This membrane can always be shown by examining a 
fragment of one of these septa with the microscope, while 
it is acted upon by very diluted hydrochloric acid; but 
very frequently it can be found only partially or very 
slightly covered with coalescing particles of carbonate of 
lime. And this is the most favorable condition under 
which it can be examined. Dr. Carpenter says, that he 
has ascertained that the nacreous lustre is due to the pli- 
cation, or folding of a single layer of this membrane in 
such a mode that the folds shall le over one another in 
an imbricated manner. Although Dr. Carpenter’s evi- 
