Royal Society. 483 



It follows from this examination :^- 



lst. That metallic copper is the most delicate and surest re- 

 agent for the detection of arsenic, the reaction taking place 

 with a solution containing 1 -200,000th part. 



2nd. That with antimony the action is similar to arsenic, 

 but it is not precipitated with such a metallic lustre, and dif- 

 fers from it by the decided violet colour produced. 



3rd. That lead and tin are not precipitated either from a 

 concentrated or dilute solution in the metallic state ; this 

 only occurs upon exposure to the air. 



4th. The solution of bismuth is immediately precipitated 

 in a crystalline state. 



5th. That silver and mercury are precipitated with a silver 

 lustre, and the reaction does not exceed a l-20,000th part in 

 solution. 



LXXIII. Proceedings of Learned Societies, 



ROYAL SOCIETY. 

 [Continued from vol. xviii. p. 241.] 

 June 17, r T , HE following papers were read, viz. — 



1841. A 1. "Experiments on the electric conditions of the 

 Rocks and Metalliferous Veins (Lodes) of Longclose and Rosewall 

 Hill Mines in Cornwall." By William Jory Henwood, Esq., F.R.8., 

 &c, Secretary of the Royal Geological Society of Cornwall. 



The experiments, of which the results are given in this paper, were 

 undertaken with the view of determining whether it was in conse- 

 quence of the imperfections of the galvanometers, or other apparatus, 

 employed, that Mr. R. W. Fox, and other experimenters, had been 

 unable to detect the presence of electricity in the tin veins of Corn- 

 wall. The mode of experimenting was in principle the same as that 

 pursued by Mr. Fox, namely, that of placing plates of metal in con- 

 tact with the points to be examined, carrying wires from the one to 

 the other, and interposing a galvanometer in the circuit. The plates 

 employed were of sheet-copper and sheet-zinc, and they were about 

 six inches long, and three inches and a half wide. The wires were 

 of copper, one twentieth of an inch in diameter, and the same that 

 had been used by Mr. Fox. 



The tabular results of these experiments show that both the gra- 

 nite and the tin vein at Rosewall Hill mine, and also the greenstone 

 and the copper vein in that of Longclose, present unequivocal traces 

 of electric currents, whether different parts of the same veins or va- 

 rious portions of the same rocks were examined. 



It also appears, from these experiments, that the nature and posi- 

 tions of the small metallic plates employed materially affect, not only 

 the intensity, but in some cases also the directions of the currents ; 

 and also that there is a considerable difference in the results when the 

 same plates of metal are placed on different ingredients in the veins, 

 even though these may be in immediate contact with each other. 



2l2 



