' and Properties of TabasJieer. iSigll 



place the air from its pores. These results are perfectly ana- 

 logous to those which we obtain with hydrophanous opal, and 

 I have also succeeded in giving transparency to the chalky 

 silex from the Gianfs Causeway, by long immersion in oil of 

 beech-nut. 



If, instead of immersing the tabasheer in water, we place a 

 small drop upon the most transparent variety, the drop is in- 

 stantly absorbed, but the spot which it occupies becomes as 

 white and opaque as if it had been covered with white lead. 

 This extraordinary property, which is not possessed by any of 

 the siliceous minerals, will be explained when we have treated 

 of the optical properties of this substance. 



The opaque tabasheer which has become transparent by 

 absorbing oil exhibits a very curious phenomenon by change 

 of temperature. If it is laid upon a piece of cold lead, it be- 

 comes suddenly opaque, and if it is restored to a warmer si- 

 tuation, its transparency as suddenly returns. These effects 

 obviously arise from the great expansion and contraction of oil 

 by heat. When the oil retreats from the surface of the speci- 

 men, the mutual attraction of its own particles accumulates 

 them in one place, instead of permitting them to remain in a 

 state of contraction in separate pores, as might have been ex- 

 pected. When the greater part of the oil has been expelled 

 from these specimens by heat, the tabasheer exhibits a beauti- 

 ful veined structure, the veins being sometimes parallel, as in 

 the onyx, and sometimes curved, as in the agate. This effect 

 arises from the different degrees of porosity in the different 

 veins, in virtue of which some of them absorb more oil than 

 others. The limits of each vein are thus rendered visible in the 

 very same manner as the veins of burned chalcedony, which 

 has absorbed oil from the lapidary's wheel, may be displayed 

 in all their beautiful inflexions, although in its natural and 

 transparent state it did not exhibit the slightest trace of such 

 a structure. It is from the same property of some of the 

 amorphous siliceous minerals that the lapidary is able to de- 

 velope, and to colour, the veins of particular agates, and that 

 the artist can execute the finest drawings, which actually lie 

 beneath the surface of certain porous specimens of chalce* 

 dony. 



