34)4' Dr. Joseph Reade 0)i Electro-Itotatory Poi?2ts, 



in manganesic oxide of iron (Jacotinga), and the rock is schist- 

 ose specular iron ore ; and at Catta Preta, where the vein- 

 stone is quartz, and the country quartzose sandstone. At 

 Gongo Soco the shoots dip eastward from the granitoid rocks 

 of the Serra de Tejuco, whilst at Catta Preta they incline 

 westward from similar rocks near Inficionada. 



There is not only a remarkable similarity between these 

 facts and the disposition of the shoots of ore in Cornwall and 

 Devon*, but the striae present a second class of phoenomena, 

 which are either less conspicuous f or altogether wanting there. 



There are some remarkable analogies between the arrange- 

 ment of gold in Brazil and that of tin ore in Cornwall, but I 

 reserve their discussion until another opportunity. 



The auriferous rocks of Brazil afford but few data for as- 

 signing their geological epoch ; there is, however, much pro- 

 bability that they may eventually be found the equivalents of 

 the metalliferous formations of the West of England. 

 Gongo Soco Gold Mines, June 29, 1844. W. J. Henwoqd. 



LVIII. On Electro-Rotatoiy Points^ and the Pyro-Electricity 

 of Glass. By Joseph Reade, Esq.^ M.D. % 



IN the year 1750 Dr. Franklin made a number of experi- 

 ments to ascertain the attractions and discharges of the 

 electric force from points, but as these points werefxed, such 

 as a needle held in the hand before a conductor, or a shoe- 

 maker's punch under a pair of excited scales to represent the 

 clouds, Dr. Franklin says " he could not account for the at- 

 tractions." If the scales were electro-positive, the punch must 

 have been electro-negative. Rotatory points were first used in 

 1660 by Dr. Gilbert (described in his work De Magnete). This 

 instrument at present, with some little alterations, is known as 

 Haiiy's electroscope, and consists of a slender wire revolving 

 on a pivot, and armed at both ends with gilt pith-balls. In 

 the year 1785, a celebrated French philosopher. Coulomb, in- 

 vented the balance of torsion, and turned his attention to elec- 

 tric points, but as they werefxed, like those of Franklin, the 

 same objections remain. Indeed the only revolving points 

 were the electric fly and the electric orrery, and in these the 

 revolutions were erroneously attributed to recoil between the 

 air and the electric force when discharged, — a physical impos- 

 sibility. An imponderable acting on a ponderable substance 

 is unknown in the science of statics ; as well might light recoil. 



* Cornwall Geol. Trans., vol. v. p. 193. Edin. New Phil. Journal, vol. 

 xxii. p. 137. t Cornwall Geol. Trans., vol. v. p. 182. 



J Communicated by the Author, through Mr. E. W. Brayley. 



