MR. HORNER AND SIR DAVID BREWSTER ON 
52 
My DEAR Sir Belleville, January 1st, 1836. 
In the communication which I had the pleasure of addressing to you on the 20th 
of January 1835, I gave a brief account of the observations I had made on the highly 
interesting substance which you had put into my hands ; but as the specimens which 
you sent me a few days ago are so much superior to those with which I made my 
former experiments, and have led me to some new and I think rather extraordinary 
results, I shall include in the present letter all my former observations. 
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 iridescent, and in very thin plates : it is almost perfectly transparent, 
with a slight yellowish brown tinge like plates of glue or lac of the same thickness. 
The laminae of which it is composed 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 adhering plates are separated, the separated surfaces are sometimes 
colourless, especially when these surfaces are corrugated or uneven ; but they are 
almost always covered with an iridescent film of the most brilliant, and, generally, 
uniform tint, which exhibits all the variety of colours displayed by thin plates or 
polarizing laminae. 
The substance is of intermediate hardness between calcareous spar and sulphate of 
lime. It scratches the latter easily, and is not scratched by mother-of-pearl. Its 
specific gravity is shown in the following Table, which indicates its relation to analo 
gous substances. 
Calcareous spar 2*72 
Oriental pearls 2‘68 
New substance 2‘44 
Mother-of-pearl 2T9 
Oyster-shell 2‘02 
The new substance has the property of refracting light doubly, like most crystal- 
lized bodies ; and, as in agate, mother-of-pearl, & c., one of the two images is perfectly 
distinct, while the other contains a considerable portion of nebulous light, varying 
with the thickness of the plate and the inclination of the refracted ray. It has one 
axis of double refraction, like calcareous spar, which is negative, as in that mineral, 
and, like it also, it gives a beautiful system of coloured rings by polarized light. The 
double refraction of the substance is very considerable, though greatly less than that 
of calcareous spar. A plate, one seventy-fifth of an inch thick, makes the first red 
ring of the system eight inches in diameter at a distance of twenty-six inches from 
the eye. The substance belongs to the rhombohedral system, and, as in the Chaux 
carbon aide basde of Hauy, the axis of the rhombohedron, or that of double refraction, 
is perpendicular to the surface of the thin plates. As mother-of-pearl has two axes of 
