color
between them are noticeable. The material occupying the space between
the rings is the sectional appearance of the skin of pearl. Upon
applying a weak acid to the surface of an entire section of a pearl, it
effervesces, and the inner colorless parts of the bands are at once
attacked. After several hours the white inner part of the skins will
show depressions where the calcium carbonate has been dissolved, and
the outer parts of the skins will be marked by coarse black rings of
undissolved animal tissue, similar in appearance to the epidermis of
the shell. Now as these skins are made up of many very thin waves of
calcium carbonate lapping each other and set in animal tissue, it would
appear, therefore, that in the beginning these waves of transparent
calcium carbonate are set in animal tissue of extreme tenuity and that
the proportion of animal tissue increases with the growth of the skin
until it reaches a stage provocative of a new skin, which begins with
purer layers of the smoother crystallized mineral like its predecessor,
and identical with the nacre of the shell. If this be so, it would
account for the various tints of color and degrees of
150
