140 Account ^Messrs Perkins and Co’s. Invention for 
lour, and its crystalline form. The very minute portion of the 
ore which was in my possession, rendered it impracticable to 
remove this uncertainty ; but in order to ascertain its ingre- 
dients as far as possible, I sent all the fragments I could spare 
to Dr Fyfe, who informed me on the 6th of March, that the 
difficultly soluble powder appeared to be sulphate of lead, and 
that the mineral was likely to be a sulpho-carhonate^ although 
it was necessary to subject a greater quantity to examination be- 
fore this could be decidedly ascertained. On the other hand, it 
is proper to state, that in another specimen belonging to Mr 
Allan, of the same colour, the same crystalline form, and the 
same optical properties, Dr Fyfe detected traces of muriatic 
acid. 
With the prospect of removing this uncertainty, Mr Laing 
Meason and Professor Irving had the goodness to promise, that 
they would endeavour to procure some specimens of this mine- 
ral from Wanlockhead and Lead Hills. Professor Irving has 
succeeded in obtaining a very fine specimen, which he himself 
examined, and found to be a sulpho-carhonate^ — a result which 
has been completely confirmed by Dr Fyfe. This specimen, 
which he was so good as to present to me, is a sort of geode, 
from which irregularly crystallised prisms shoot up in various 
directions. One of these prisms is about an inch and two-tenths 
long, and several about three quarters of an inch. Their co- 
lour is a sort of bluish-grey. Other flat rhomboidal crystals, 
like those from Wanlockhead, with bevelled edges, occur among 
the prisms, but their form cannot be easily determined. The 
surfaces of the prismatic crystals were too iri'egular to be mea- 
sured ; but I found that they were rectangular, with additional 
faces, some of which were incident at angles of 120®, and others 
at 145®, to the adjacent planes. 
Aet. XXV. — A Brief Account of Messrs Perkins, Fairman^ 
and Heaths Invention for Perpetuating Engravings upon 
Steel, or other Metals. » With a Plate. 
The present age has perhaps produced a greater number of 
inyentions of real value, and more importance to the civilised 
