A SUBSTANCE RESEMBLING SHELL. 
55 
The iridescent phenomena exhibited by the new substance are extremely interest- 
ing, and I have been at much pains to examine them in a great number of specimens. 
The plates into which the substance is divisible have been formed in succession, and 
certain intervals of time have elapsed between their formation. In general every two 
contiguous laminse are separated by a thin iridescent film, varying from the three to 
the fifty millionth part of an inch in thickness, and producing all the various colours 
of thin plates which correspond to intermediate thicknesses. Between some of the 
laminae no such film exists, probably in consequence of the interval of time between 
their formation being too short ; and between others the film has been formed of un- 
equal thickness, as happens in the oxidations upon steel when they are formed upon 
or around hard parts of the metal called pins by the workmen. 
There can be no doubt that these iridescent films are formed when the dash-wheel 
is at rest during the night, and that when no film exists between two laminae, an in- 
terval too short for its formation (arising perhaps from the stopping of the work 
during the day,) has elapsed during the drying or induration of the one lamina and 
the deposition of the other. 
That these iridescent films are not thin films of the substance itself, may be inferred 
from the fact that light is reflected from their surfaces when they firmly adhere to 
the laminae which inclose them. If, for example, we remove or raise up from a piece 
of mica a thin film which gives a bright green tint, and press it again into optical 
contact with the surface from which it was separated, it will then cease to exhibit 
any colour, because no light is reflected from its posterior surface ; but if we press it 
into optical contact with another surface which has a different refractive power, its 
green colour will still be exhibited. It is owing to this cause that the colours of the 
oxidations on steel are so distinctly visible, and that the analogous oxidations are 
seen upon glass even before the film has begun to separate into coloured scales. 
The iridescent films in the new substance possess another source of interest, in so 
far as they promise to throw a new light on the origin of the incommunicable colours 
of mother-of-pearl, which arise from the interior structure of the shell, and which 
cannot therefore be communicated to wax. These colours have frequently occupied 
my attention since the year 1814, when I described the phenomena of the colours 
communicable to wax *; but though I have devoted much time to the inquiry, I 
never could obtain a single result worthy of being communicated to the public. I 
took plates of mother-of-pearl that exhibited different bright colours through different 
parts of their surface, and by getting the mother-of-pearl ground away in different 
places by the seal -engraver’s wheel, I endeavoured to discover the thicknesses at 
which the colours were produced, and the cause of the capricious variation of tints 
which arose from every inclination of the plate : but all my experiments were fruit- 
less, and I abandoned the subject as beyond my reach. The phenomena, however, 
presented by the new substance seem to me to disclose the secret of which I was in 
* Philosophical Transactions, 1814. 
