Professor Leslie on Electrical Theories, W 
to the difference between the proximate intensities. And the 
total effort to destroy the texture of the conductor depends 
partly on its narrowness and slow communicating quality, but 
much more on its shortness. 
Art. II . — Experiments on the Growth of Pearls, with Obser- 
vations on their Structure and Colour. By W . T. 
Pearls, in general, take the colour of the shell in which 
they are formed, being nothing else than the substance of the 
shell disposed in concentric layers, and tending more or less to a 
spherical form. From the great number of small pearls which 
I have frequently collected from the small sea-mussel ( Mytilus 
edulis ) so common in the streets of London, I find that there is 
no part of the flesh of this animal in which pearls do not occur. 
The natural expenditure of the substance which forms the pearl, 
is only for the purpose of producing the shell or testaceous co- 
vering of the shell-fish ; but various causes producing wounds in 
the animal, or otherwise irritating it, will produce a secretion of 
•the shelly matter to defend the injured part ; and however sharp 
or angular the offending substance may be, it by degrees as- 
sumes a round form, in proportion as it is covered by a greater 
number of coats. The assertion advanced by Linnaeus, and re- 
peated in some works, that the Chinese have a mode of pro- 
ducing by art real pearls in the living shell-fish though in ge- 
neral little credited, seemed to me so feasible, that I was led to 
attempt a similar experiment, which I tried upon the large mus- 
sel of our ponds ( Anodonta cygnea ■), being the only convenient 
shell-fish winch I could command in the central parts of Eng- 
land. I procured, therefore, the largest of these, from five to 
six inches long, from the Duke of Marlborough's Water of 
Blenheim ; but although the vigour of the animals promised me 
* Either in Hunter’s Museum, or in that of the late Duchess of Portland, was 
to be seen a pearl-shell, said to be from China, containing a string of pearls, ap- 
parently produced in the manner referred to ; which is that of introducing iron- 
wires at given distances through the shell of the animal, so as to irritate its flesh, 
without entering deep enough to kill it. 
