36 FORMATION OF SHELLS OF ANIMALS, ETC., 
specified, the uppermost part of the deposit will exhibit 
perfect crystals, that immediately beneath it crystals be- 
ginning to have their angles rounded off, and the exami- 
nation thus continued successively upon still lower por- 
tions, will show the gradual passage of imperfectly recti- 
linear figures into forms perfectly spherical. If, on the 
contrary, the density of the lower or alkaline solution be 
not sufficient, the smaller globular particles will fall to the 
bottom of the bottle before they have had time to coalesce 
m sufficient quantities to form the larger calculi. But 
when the densities of the two solutions are properly pro- 
portioned, as in the formula before given, no crystalline 
carbonate will be found adherent to the upper part of the 
slide, but only globular carbonate of lime; lower down 
there will be the latter compound, triple phosphate, and a 
mixture of the two. This mixture will be in the globular 
form, and presenting different appearances according to 
the relative proportions of the two component ingredients ; 
if there be an excess of triple phosphate, the surface of 
the calculi will be studded with minute bright crystals, 
but if an excess of the carbonate no such appearance is 
present, but their surface will be smooth, and their inte- 
rior more or less finely lamimated. At some distance 
below these nothing but crystals of triple phosphate will 
be seen. Now, the examination of these crystals in refer- 
ence to the gradual change their form undergoes is very 
remarkable. Beginning with the lowest and proceeding 
upwards, these crystals will be seen to be at first beauti- 
fully perfect, presenting angles and edges sharply defined 
and perfectly rectilmear. Next, as they are examined 
higher on the slide, where they begin to mix with the cor- 
bonate of lime, they will be observed gradually to lose 
their rectilinear and angular form, and to become irregu- 
