112 FORMATION OF SHELLS OF ANIMALS, ETC., 
ing their convexity. In Dr. Carpenter’s plate, represent- 
ing a portion of the shell of Pinna, two small spaces, 
situated at the conflux of several compressed globules, are 
represented dark. In these the calcareous matter is said 
to be deficient, and the dark appearance, as before ob- 
served, is attributed to the presence of air. This appear- 
ance is fairly represented in Dr. Carpenter’s delineation ; 
but the cause he assigns for it is not, I am persuaded, the 
right one, being unconnected altogether with air. It 
is caused by the particles of globular carbonate of lime, 
in which this appearance is observable, bemg both more 
dense in structure, as well as smaller than those to which 
they are contiguous, and consequently more highly re- 
fractive ; hence the smaller, that is, the more convex these 
particles are, the darker they appear. Ifa single layer of 
these compressed globules be viewed by reflected lght 
with the superficial or outer surface uppermost, the small 
particles, looking the darkest by transmitted light, will 
then appear to be of the densest white. The smallest, or 
dark particles, are situated at the conflux of several larger 
ones, and generally deeper, but not always. They present, 
therefore, great difference in their shade and in their appear- 
ance, accordingly as, when viewed, they are superficial 
to or deeper than the contiguous particles. Now, these 
particles being, from their greater density and smaller 
size, more highly refractive than the contiguous ones, an 
appearance is produced resembling that caused by the 
presence of air; this appearance being imcreased by 
the unequal refraction of the globules through which 
these smaller ones are seen. But, it may be observed, 
that, as the appearance of air in these particles is only ob- 
servable when they are seen with one of their sides next 
to the microscope, it cannot be occasioned by the presence 
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