On the EltctricUy of Minerals. 83 



To proceed to the experiments: let us suppose in the 

 tirst place that we wish to dcterniine the positions of the 

 electrical poles of a tourmaline, which is of a thin and ob- 

 long form, like all tlie tourmalines of Spain. After having 

 heated it, we shall place it in the apparatus, and present 

 successively, at a small distance from its two extremities, 

 another body which has been electrized by friction. I 

 prefer employing for this purpose the Saxon or Brazil to- 

 pazes, because these minerals, as I have remarked in my 

 Treatise on Mineralogy, are extremely sensil)ie to the action 

 of friction in order to excite the electrical virtue in them, 

 so that not only a damp aimosphere docs not hinder them 

 from acquiring it, hut they preserve it for a considerable 

 time. The kind of electricity in question being of the 

 same nature with that of glass, the pole of the tourmaline 

 which the topaz will repel will be the vitreous pole, and 

 that upon which it will act bv attraction will be the resinous 

 pole. It is sufiicient to have a tourmaline susceptible by its 

 form of being placed in the apparatus, and the poles of 

 which are known, in order that it may serve as a term of 

 comparison to all the bodies of the same species, or of dif- 

 ferent species, which' share the property in question, what- 

 ever in other respects are the forms and dimer.sions of such 

 bodies. After having heated that which we wish to exa- 

 mine, we bring it successively by its two extremities near 

 both of those of the tourmaline, and the consequence of the 

 result is presented of itself, agreeably to the principle com- 

 mon to electricity and to magnetism, viz. the pole soli- 

 cited by homogeneous fluids is repelled, and those in which 

 heterogeneois fluids reside are attracted. 



The advantages of the apjiaratus which I have described, are 

 particularly lelt in the experiments relative to borated magne- 

 sia, which, in order to succeed, require very favourable cir- 

 cumstances, vvfhen we use the metallic ntedle which I first 

 mentioned. It is necessary that we should wait until the ac- 

 tion of the tourmaline placed in the apparatus has been dimi- 

 nished, by cooling, to the point of being found in proportion 

 to the i'ecble virtue of the crystal of borated u^-agnesia; and 

 we must also take care to keep the crystal in such a position, 

 that, the axis which ])asses by tlie pole uhich we present to 

 the tourmaline being perpendicular to the length of the 

 latter, the same pole corresponds to the centre of action of 

 the tourmaline, which we. know to be Very near the ex- 

 tremity. 



This apparatus may be also employed for determining tliii 



kind of electricity wiiich a body acquires by means of fric- 



F 2 tion. 



