357 
Another experiment was made on one of those varieties of the 
opal which are known by the names hydrophanes, changeable opal, 
and oculus mundi, which, besides possessing the remarkable property 
of becoming transparent in water, and other fluids, are eminently 
distinguished by manifesting, in that situation, the beautiful play, 
or change of colours, peculiar to the opal. This analysis was made, 
under those circumstances, which were calculated to yield some 
knowledge of the nature of those constituent parts, which had been 
dissipated, without examination, in the two experiments just men- 
tioned ; and therefore is the experiment, on which we might, almost, 
be allowed to depend the decision of the question — whether any 
thing of a bituminous nature enters into the composition of the opal. 
The result of this experiment is strongly affirmative of this fact ; 
and, indeed, seems to confirm the opinion I have ventured to ad- 
vance, in a most striking manner. You must have perceived that I 
have been regularly led on to the supposition of the presence of 
bitumen in opal, by first noticing the impregnation of bituminous 
wood with silex, by which was formed silicized bituminous wood ; 
then tracing this into the opalized wood and the pitch-stone ; thence 
to the semi-opal, and at the last to the opal itself ; you will there- 
fore readily conceive the satisfaction yielded by the analysis made 
by Mr. Klaproth, which afforded a result at once so unexpected 
and decisive ^ and so precisely accordant with the experiments of 
Mr. Pepys, on opaline wood. 
One hundred grains of the hydrophanous opal of Saxony were 
coarsely divided, or bruised, and ignited upon red-hot coals, in a 
small glass retort. At the end of this process there appeared, in 
the receiver, an empyreumatic water, covered with a thin greasy pel- 
licle. The loss of weight arising from this was 5^ grains. Pursuing 
the analysis, Mr. Klaproth was enabled to ascertain that 100 parts 
of the hydrophanous opal, from Saxony, contain. 
