Geology. Artificial Colours in Agate.

The Year-Book of Facts in Science and Art
Exhibiting the Most Important discoveries and Improvements of the past year,
in mechanics and the useful arts; natural philosophy; electricity; chemistry; zoology and biology; geology and geography; meteorology and astronomy.
By John Timbs,
editor of "the Arcana of Science and Art."
London:
David Bogue, Fleet Street,
MDCCCXLIX (1849)

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* Mr. Hamilton believes that not a few of the agates which have come down from ancient times have been thus treated.Tar change of Colour produced Artificially in the Agates by the workers in them at Oberstein, an art learned from the Italians, is of much interest mineralogically, since it shows the very different porosity of different layers in the agates, the least porous bands not being necessarily the nearest to the centre, but dispersed irregularly through the mass. To this porosity Mr. Hamilton calls attention, citing the researches of M. Noeggerath, who states, that in some layers the minute hollows can be seen by means of a magnifying glass; that, while some are round, others are long, and that they sometimes run into one another. These hollows, Mr. Hamilton considers, may form interstices between the radiating crystals. By immersion for some time in honey and water or olive oil, so that the pores of the agate become more or less filled with a substance to be carbonised, a subsequent soaking of the stone in sulphuric add produces a difference in the tints of the agate according to the porosity of the layers, the most porous becoming black, while the least porous remain white or uncoloured. By immersion in a solution of sulphate of iron, and a subsequent heating of the agate, a cornelian red is in like manner obtained for the most porous layers, the iron being converted into a peroxide, while the least porous layers continue unchanged in colour.* It would be out of place further to dwell upon the infiltration of mineral matter in solution into the isolated cavities of rocks. The mode in which the various minerals occur is highly interesting, as also their connection with the matter filling veins and fissures in adjoining parts of the same or adjacent rocks, as, for example, the filling of the fissures in the red conglomerate by the same kind of siliceous matter which entered into the cavities of the igneous rocks of Idal, the layers having, in both cases, adjusted themselves to the surfaces on which they were accumulated. — Sir Henry T. De la Beche's Anniversary Address to the Geological Society.