396 Mr. C. Tomlinson on some Effects of small 



Oberstein, or merely brought there to be cut and polished, is 

 another question. It subsequently became known that large 

 numbers of these agates were imported from Brazil and other 

 places. 



The effect of small portions of foreign matter in modifying 

 the structure and physical properties of siliceous minerals is 

 a difficult subject, and often one of no little embarrassment to 

 the mineralogist. In agate, for example, there may be 98 

 per cent, of silica, and minute portions of iron, in zigzag lines, 

 as in the variety known as fortification agate ; in other 

 specimens, in various forms of animals and plants. These 

 lines seem to be the edges of successive layers of material 

 during the process of formation. Advantage has been taken 

 of these lines to stimulate the artist to complete some of them 

 into the forms of natural objects, forming what the French 

 call agates zoomorphytes. These curiosities were in demand 

 among the ancients. Thus Pliny (Lib. 37, Cap. 75, &c.) 

 refers to three methods of manufacture, and describes an 

 agate, the natural markings of which represent Apollo and 

 the Nine Muses. In the British Museum there is a globular 

 or Egyptian Jasper, which exhibits in the two fractured sur- 

 faces a likeness of the poet Chaucer. There is no reason to 

 suppose that this has been doctored, but of course there is a 

 strong temptation to fabricate these trifles, since they fetch a 

 high price. Brard, in his Mineralogie appliquSe aux Arts, 

 describes how chalcedony may be etched with a metallic 

 solution, such as blue vitriol in aquafortis, the result on drying 

 being corroded lines of a brown colour. But he adds that 

 designs produced in this way have a more finished appearance 

 than the natural markings. Italy still continues to be the 

 seat of this manufacture, as it probably was in Pliny's time. 



Various devices are adopted at Oberstein for conferring an 

 ornamental value on agates and other pebbles, whether native 

 or imported. In one process the cut stone is covered with a 

 layer of carbonate of soda, and heated to redness in a muffle. 

 This produces a white opaque enamel, as hard as the stone 

 itself, and adapted to cameo cutting. A similar result was 

 produced during the great fire at the Tower of London some 

 years ago, when many of the gun-flints encountered a rain 

 of fused nitre, and so became encrusted with white enamel. 

 Advantage is taken of the banded structure of some of the 

 pebbles to colour the amorphous quartz artificially. The 

 pebbles are washed and dried and then placed in an aqueous 

 solution of honey or of sugar, about half a pound to a pint. 

 The vessel is then put on a stove, or in hot ashes, and fresh 

 liquid is supplied from time to time, so as to keep the stones 



