26 FORMATION OF SHELLS OF ANIMALS, ETC., 
composing the superficies, and diminishing in the ratio 
of the distance from the centre, its effects would of ne- 
cessity be apparent, first at the surface of the calculus, 
and last at the centre. Hence, according to this ap- 
plication of the laws of gravity, the most external layer 
will be the one first formed. Now, with respect to 
the subsequent layers it may be observed, that, as the 
ultimate effect of gravity upon the molecules of matter is 
so to arrange them that they shall occupy the least possible 
space, and as the exterior molecules have been shown to 
be thus arranged whilst the interior remam im their 
amorphous condition, it must follow, that when the former 
have undergone their final arrangement, that is, are 
brought into the smallest compass, the latter will be insuffi- 
cient completely to fill up the space enclosed by the outer- 
most lamima, and therefore an interval will exist between 
them. But the microscopic examination of such calculi 
during their coalescence and lamination shows that this 
process is not divided into two separate and distinct 
stages, as might be inferred from this explanation (which 
has only been made to present this idea to show more 
clearly the facts), but that it takes place at several suc- 
cessive periods, and consequently as many layers, as there 
are periods of formation, are produced, one layer being 
within the other. Now, as the inductive reasoning de- 
monstrative of the existence of a space between the mole- 
cules, when complete coalescence has been twice effected, 
will apply equally if this coalescence be effected at any 
number of separate times, it must follow that, as the first 
jayer is larger than necessary to contain the second, 
this in the same way will be too large for the third, and 
so on for the rest; and hence, that there will result in- 
tervals or spaces between the laminz, whatever may be 
