Chap. 14.] Lapis Nephriticus, Cat's Eye, &c. 9t 



colour, are the vulgar lapis nephriticus ; which name 

 they have obtained, from a fuppofed property of cur- 

 ing nephritic pains, when applied externally to the 

 loins. Their femipellucid appearance, hardnefs, and 

 fpecific gravity, are characters by which the lapis neph- 

 riticus may be diftinguifhed from other ftones. 



2. The cat's eye is a very fcarce (lone. It is 

 opake, and reflects green and yellow rays, in a man- 

 ner fomewhat fimilar to the eye of the animal from 

 which its name is derived. It is found in Siberia. 



3. The hydrophanes, or oculus mundi. The cha- 

 racter which diftinguifhes this from all other ftones is 

 its property of becoming transparent in confequence 

 of being immerfed in water. This happens from its 

 imbibing that fluid, as it becomes again opake by being 

 dried. 



This circumftance may be illuftrated by a compan- 

 ion with fome other natural phenomena. The appear- 

 ance of clouds ferves to mew that particles of water, 

 copioufly mixed with thofe of air, prevent the tranf- 

 miflion of light, and render the mafs more or lefs 

 opake, although the particles of each, feparately taken, 

 -are pellucid. The caufe of this is to be fought for in 

 the difference of the refracting power j for, by means 

 of that, the rays are turned from their original courfe 

 at every particle, a circumftance which is very inju- 

 rious to tranfparency. A common experiment will 

 ilill more clearly elucidate this point. The moft 

 tranfparent glafs, when reduced to powder, becomes 

 opake, becaufe the air which now occupies the inter- 

 flices reflects the light much more copioufly than the 

 particles of glafs. But upon pouring on water, which 

 difplaces the air, and which differs lefs from the glafs 

 in its refracting power, a certain degree of tranfpa- 

 rency is reftored. On this principle depends the na- 

 ture 



