S42 
Dr MacCulloch an the Method 
agates by artificial means. It would be necessary to re-examine 
a collection of these substances, and to repeat some of the expe- 
riments on them, to determine the extent to which this art may 
be carried, and the exact nature of the varieties which are sus- 
ceptible of the changes in question. As the discovery of the in- 
ternal structure of agates is your own, no one is more compe- 
tent than yourself to make these further inquiries, from which I 
am at this moment precluded. 
It has long been known, that zoned agates, formed of laminae 
alternately black and white, were brought from India ; but it is 
only since the peace that the same substances have been import- 
ed from Germany in considerable quantity ; in consequence of 
which their value has fallen to little more than the price of cut- 
ting. These latter are coloured by an artificial process, which 
is a kind of secret in the trade, and it is not improbable that the 
specimens from India are produced in the same manner, as the 
natives of that country possess the art of staining the same mi- 
nerals white. As the lapidaries are not acquainted with the 
theory of their process, they are very subject to failures, which 
also arise at times from the nature of the stones being unsus- 
ceptible of the colouring process. 
The common process consists in boiling the cut specimens in 
sulphuric acid ; in consequence of which, a particular lamina, 
or set of laminae, is rendered black, while others retain their na- 
tural colour, or even become whiter than before, thus produ- 
cing that povrerful contrast which is esteemed valuable in this 
stone. This process often fails, and will always fail, if tried on 
specimens which have not previously been cut on the lapidaries’ 
wheel. It is, in fact, produced by the action of the sulphuric 
acid on the oil which has been absorbed by the stone in cutting, 
and can therefore very obviously be insured, by previously 
boiling in oil the specimens which are to be subjected to the 
blackening process. That this is the fact is proved, if proof 
were necessary, by the disengagement of sulphurous acid gas, 
which takes place during the action of the acid. To insure suc- 
cess, therefore, it is evident that either the application of the oil 
must be continued for a sufficient length of time, or that the 
stone be cut so thin as to admit of its being penetrated by it before 
the sulphuric acid is applied. You will easily see, that this ab- 
