an artificial Substance resembling Shell. 20^ 



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 phaenomena of the colours com- 

 municable to wax*; but though I have devloted much time to 

 the inquiry, I never could obtain a single result worthy of be- 

 ing 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 atVhich 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 fruitless, and I abandoned the subject 

 as beyond my reach. The phaenomena, however, presented 

 by the new substance seem to me to disclose the secret of 

 which I was in quest. The layers of mother-of-pearl are de- 

 posited in succession like those which are formed upon the 

 dash-wheel ; and there can be no doubt that the animal whose 

 mucous secretions form the shell that incloses it, rests occa- 

 sionally from its toils, and affords a sufficient interval for the 

 formation of an iridescent film upon the surface of the plate 

 of shell which it daily deposits. Owing to the firm adhesion 

 of the successive layers of the shell, we cannot, as in the more 

 imperfectly formed new substance, separate each stratum in 

 order to see the iridescent film upon their surfaces ; but we 

 can easily determine what phaenomena would be produced if 

 the layers of the new substance were as transparent as those 

 of mother-of-pearl. If this were the case, we should see, 

 both by reflected and transmitted light, the combined colours 

 of all the iridescent films in the plate. When these films 

 are numerous and flat, and of various thicknesses, the union 

 of all their colours would form a pearly whiteness by re- 

 flected light, and when films of a particular colour predomi- 



» Philosophical Transactions, 1814. 

 Third Series. Vol. 10. No. 60. March 1837. 2 E 



