Colouring Agates artificially. 343 



sorbent property of agates explains the fact of their being occa- 

 sionally blackened by sulphuric acid, so as to have led chemists 

 to imagine that they naturally contained some carbonaceous 

 matter ; and it was a knowledge of this circumstance which 

 led me to subject to long boiling in a solution of potash, 

 those specimens in which I suspected vegetable remains to be 

 entangled, before applying to them the test of sulphuric acid. 



The fact itself is curious in another point of view, as it indi- 

 cates the porosity of agates, and is nearly connected with your 

 interesting discoveries respecting their internal structure. In 

 examining the specimens which are to be subjected to this treat- 

 ment, it is in the first place evident, that the future changes 

 are not indicated by the colours, as the red, or otherwise colour- 

 ed laminae, sometimes become black, while in others the opaque 

 white, or the transparent* are affected. In the few specimens 

 which I possessed at the time I was engaged on this subject, I 

 had no means of determining in what respect the change was 

 connected with apparent differences in the internal structure, and 

 I can only now suggest it to you as a subject for examination. 

 In most zoned agates, some of the laminae will be found exempt 

 from any apparent internal structure, while in others the ap- 

 pearance of undulating fibres is evident, the fibres themselves 

 varying materially in size. If any conjecture were to be form- 

 ed a priori, it would be expected that the most distinctly fib- 

 rous laminae were the most porous; but it is unnecessary to 

 offer conjectures on what you may so easily put to the test of 

 experiment. 



Having mentioned the Indian practice of colouring agates 

 white, it may be remarked, that this is also a secret in the hands 

 of lapidaries, although apparently not generally known to them, 

 and but little practised. Beads of carnelian are sometimes 

 brought from India, ornamented with reticulations of a white 

 colour, penetrating to a small depth within the stone, and 

 equally hard. The black agates of this kind, which are some- 

 times coloured with complicated or fine lines of white, are often 

 very singular, and, without a knowledge of the mode in which 

 they are produced, have a very puzzling appearance. 



They are thus coloured by applying carbonate of soda, and 

 exposing them to the heat of a furnace or a muffle. An opaque 



