342 Dr MacCulloch an tJie MetJtod of 



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 powerful 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^ 



