216 ANNUAL OF SCIENTIFIC DISCOVERT. 



costly as any produced by the first jewellers in the world. The setting 

 is always of real gold, and the fashion of the newest kind. A tiara 

 from the shop of Bourguignon, of the price of six hundred francs, 

 will rival in effect and delicacy of finish its neighbor which may have 

 cost as many hundred pounds ; none can tell the difference but those 

 who have been allowed to handle it, and breathe upon it, and touch 

 it with the tongue, and apply an acid to it, in order to see whether or 

 no it becomes tarnished on the instant ! 



Really, if all this trouble becomes necessary to detect deception, 

 why, ignorance is bliss, and the distinction must be literally one with- 

 out any difference whatever. London Court Journal, 



ON A CHEMICAL CAUSE OF CHANGE IN THE COMPOSITION 



OF ROCKS. 



The following is an abstract of a paper read before the British As- 

 sociation, by Prof. Johnston. The first example of a chemically altered 

 rock adduced by the Professor, was the rotten-stone of Derbyshire, 

 a light and porous substance used chiefly for polishing metals, and 

 stated in Phillips' " Mineralogy" to be composed of silica, alumina, and 

 carbon. It is obtained from a ridge covered with "drift" 10 or 20 

 feet thick, consisting of brown clay, with manes of black marble, 

 chert, and rotten stone. The rotten-stone is so soft whilst in the soil 

 that the spade goes through it readily, but it hardens on exposure ; 

 the holes from which it is dug are sometimes only 2 feet deep, at oth- 

 ers from 6 to 8 feet. On examining a series of specimens, Prof. John- 

 ston found that whilst some were homogeneous, others had a nucleus 

 of black marble ; he then treated specimens of the black marble with 

 weak acid, and found that on the removal of the carbonate of lime, 

 there remained from 15 to 20 per cent, of a silicious substance per- 

 fectly like the natural rotten-stone. He concluded that there existed 

 in the soil some acid which penetrated it and dissolved out the calca- 

 reous matter of the rocks below. The agent in this case might be the 

 carbonic acid of the air, brought down by rain ; but there were in- 

 stances not capable of explanation by this agency alone, and attribu- 

 table to other acids, which are produced under certain conditions and 

 exercise a much wider influence. The bottoms of peat bogs present 

 very strong evidence of the action of acids, the stone and clay are 

 bleached and corroded, only silicious and colorless materials being left. 

 The source of the acid is here the same as in the former instance ; the 

 vegetable matter growing on the surface produces in its decay acid 

 substances which exert a chemical action on the subsoil, and escape 

 by subterranean outlets, carrying away the materials dissolved in their 

 progress. Another instance was afforded by the mineral pigotite, 

 formed in the caves of Cornwall by water dripping from the roof: 

 this water contains a peculiar organic acid, derived from the soil of 

 the moors, which dissolves the alumina of the granite and combines 

 with it. The organic acids are very numerous and different in com- 

 position, but agree in producing chemical action upon rocks. They 



