242 
rOPULAR SCIENCE REVIEW. 
may be well seen by comparing the mode of repair of injuries 
at the margin with that of those nearer the umbo of the shell. 
If the pearly lining be treated with a dilute acid, the car- 
bonate of lime, of which it is partly composed, will be dissolved 
out, and a structureless organic membrane will be left behind. 
The late Sir David Brewster was the first to make known that 
the rainbow-hues of this “ nacre ” were due to the presence of 
grooved parallel lines on its surface, “produced by cropping 
out of laminae of shell situated more or less obliquely to the 
plane of the surface.” He, however, supposed that there was 
an outcrop of alternate layers of membrane and calcareous 
matter ; but it has been since found out that there is but a 
single membrane disposed in folds lying more or less obliquely 
to the surface, and between which the lime-salts are deposited. 
This can be well shown by decalcifying the “ nacre ” of the shell 
of the “ ormer ” ( Haliotis ) — so common an article of food in 
the Guernsey markets — when the iridiscence will still persist ; 
but, if the folds of membrane be smoothened out by needles, it 
will disappear. 
The peculiar lustre of “ nacre” is a good example of that 
interference of rays of light reflected from different surfaces, 
which is known by the name of “ diffraction.” 
The pearls which are, or at all events, were procurable in 
the mountain-streams of Britain, from a kind of fresh-water 
mussel, Unio margaritifer ,* as well as from the better-known 
pearl-oyster of the Indian Seas, consist of layers of nacre formed 
round “foreign bodies,” which either through accident or 
design have been introduced into the shell ; for the mussel 
and oyster, like the Sybarite fretting at the crumpled rose-leaf, 
abide not the presence of even a grain of sand. 
Of this irritability of the mussel the Chinese have long 
taken advantage, by introducing, either through the outer 
shell, or between the nacreous lining and the mantle, various 
objects which they wish to become pearl-washed. In the 
Museum of the Royal College of Surgeons may be seen two 
valves of a Unio , brought from the neighbourhood of Ningpo, 
into which twenty-three plates of a white metal, each stamped 
with a figure of Buddha, have been introduced, and have been 
since coated with a layer of nacre. 
The pearls differ in quality as they are developed in the 
outer or inner layers of the mantle ; being, in the former case, 
very irregular, sometimes large, and adherent to the nacreous 
lining of the shell, while in the latter instance they are small, 
• 11 In Britannia parvas atque decolores nasci certum eat, quoniam divus 
Julius thoracem quern Veneri Genetrici in templo ejus dicavit, ex Britannicis 
margaritis factum voluerit intelligi.” — Pliny, Nat. Hist. lib. ix. 
