as exhibited in its propagation along plates of glass. 77 
Proposition XXII. 
When heat is propagated through muriate of soda,fluor spar , obsi- 
dian, semi-opal , and other minerals that have not the property of 
double refraction , they exhibit the same phenome?ia as heated 
glass . 
A mass of muriate of soda, when laid upon a hot iron, exhi- 
bited a yellow of the first order, both in the external and 
internal fringes. Fluor spar was very slightly affected. Semi- 
opal suffered a greater change; and Obsidian displayed the 
fringes as readily as glass. A piece of Obsidian of consider- 
able transparency, and about ~ of an inch thick, possessed 
naturally the fringes produced by heat. It must therefore 
have been formed by igneous fusion. This specimen, for which 
I was indebted to Mr. Sivwright, was cut out of a round 
mass, and preserved its original outline. It probably was of the 
first variety discovered by Sir George Mackenzie.* 
Rosin , gum copal, horn , amber, tortoise shell, the indurated 
ligament of the chama gigantea,f and various other sub- 
’* Sir George Mackenzie has observed, that there are two very distinct varieties 
of obsidian. One of these transmits light when cut into thin plates, which, however, 
seldom appear of an uniform degree of transparency. This variety, at a temperature 
much under that which can be excited in a common fire place by a pair of bellows, 
swells, and is converted into pumice by the extrication of a gaseous fluid, which Sir 
George Mackenzie and Dr. John Davy attempted without success to collect. 
During the experiment, the smell of nitrous acid was very perceptible. The other 
variety is denser, of the deepest black colour, and is scarcely translucent at the edges 
of thin fragments. It does not swell on the application of heat, much more intense 
than what converts the other variety into pumice. Should this note meet the eye of 
a skilful analyst, he would do a service to mineralogy by examining both varieties, and 
by comparing the analysis of pumice with that of the pumice formed from the first 
variety. 
f I have been indebted to Dr. Francis Buchanan, F. R. S. for this curious 
substance. It is as hard and transparent, and has as rich a colour a? amber. 
