204 Mr. Horner and Sir David Brewster on 



Pinna and some other shells, a prismatic crystalline arrange- 

 ment of the particles perpendicular to, and passing uninter- 

 ruptedly through, the laminae ; but I have not discovered such 

 an arrangement in any portion of the incrustation, even when 

 examined by the microscope. 



1 felt very desirous that this singular deposit should be ex- 

 amined by Sir David Brewster; the more especially as he had 

 long since directed his attention to the peculiar structure of 

 mother-of-pearl *. On showing him the specimens in my pos- 

 session he observed, that it was one of the most remarkable 

 artificial productions he had seen; and he readily undertook 

 to examine it carefully. He shortly afterwards sent me the 

 particulars of that examination, which had afforded some cu- 

 rious and interesting results. Having subsequently visited 

 Catrine, I procured more perfect specimens ; and I sent these 

 to Sir David Brewster, in order to ascertain whether they 

 might not afford something new, in addition to the results he 

 had obtained from the fragments he had formerly examined. 

 They did so, and I now subjoin the very interesting account 

 which Sir David has given me of the properties he has disco- 

 vered in this new substance. 



My dear Sir, Belville, Jan. 1st, 1836. 



In the communication which I had the pleasure of address- 

 ing to you on the 20th of January, 1835, I gave a brief ac- 

 count of the observations I had made on the highly interesting 

 substance which you had put into my hands ; but as the spe- 

 cimens which you sent me a few days ago are so much supe- 

 rior to those with which I made my former experiments, and 

 have led me to some new and I think rather extraordinary re- 

 sults, I shall include in the present letter all my former obser- 

 vations. 



The substance in question does not resemble in its general 

 aspect any natural or artificial production which I have seen. 

 It is, generally speaking, brown where the surface is not iri- 

 descent, and in very thin plates : it is almost perfectly transpa- 

 rent, with a slight yellowish brown tinge like plates of glue or 

 lac of the same thickness. The laminae of which it is com- 

 posed are sometimes separated by vacant spaces, at other times 

 slightly coherent, but generally adhering to each other with a 

 force greater than that of the laminae of sulphate of lime or 

 mica, and less than those of calcareous spar. When the ad- 

 hering plates are separated, the separated surfaces are some- 

 times colourless, especially when these surfaces are corrugated 

 or uneven ; but they are almost always covered with an iri- 

 descent film of the most brilliant, and, generally, uniform tint, 

 • Phil. Trans., 1814. 



