42 
Experiments on the Growth of Pearls, 
pearls, as well as silvery ones, produced by the Pinna marina , 
whose shell partakes of these two colours. Thus, also, we have 
in the small blue mussel of the Scottish seas, and probably of 
other parts, opaque pearls of a deep blue colour ; and I have 
one of these shells from Montrose in Scotland, in which one of • 
these blue pearls adheres, in the form of an excrescence, to the 
blue lip of the shell, whilst a dead-white one adheres to the 
white part of the shell, though without lustre, like the shell 
itself. 
A curious appearance in the flat shells of the scallop, already 
quoted, seems to confirm the foregoing observation. It is ne- 
cessary previously to remark, that these shells, when arrived at 
their full growth, consist of two colours ; the middle part of the 
shell deriving itself from the hinge outwards, is white, whilst all 
the parts towards the borders of the shell are of an obscure 
purple or chocolate colour. In this instance, there are not only 
small purple eminences or pearls rising from the purple part of 
the shell, but the same purple eminences are profusely scattered 
over the white part of the area of the shell, and in some places 
form even a veil of purple, which encroaches much on the white 
area ; whence it seems probable that the fish being advanced to 
the period of depositing a purple shelly matter, deposited it in- 
discriminately over all those parts which were then weak, and 
stood in need of repair. 
The blue pearls which are found, though not commonly, at 
Montrose, are uniformly, as far as I have seen, flattened on one 
side, where they have been in contact with the shell, which may 
probably be owing to the largeness of these pearls in proportion 
to the animal, whereas, the smaller or seed-pearls taken out of 
the same animal in the London markets are mostly round, and 
but few of them have a blue tint. Indeed, almost all the Mon- 
trose blue pearls are more or less white in some part of their 
substance. I have one of them four lines in length by two lines 
high, and as many broad, which is very large in proportion to 
the size of the animal, and must have been no small encum- 
brance to it. 
The structure of pearls is uniformly radiated from a centre, 
and consists of concentric coats, when it happens that, in break- 
ing them with a hammer, if they are not split directly through the 
i 
