[ 81 ] 



XIX. On the Electricity of Minerals. By M. Havy*. 



X HE property which certain natural bodies possess of be- 

 coming electrical by the intermedium oi' heat, furiiijhes 

 mineralocrv with one of the most advantageous characters 

 for ascertaining" what thev are, and also gives rise to ex- 

 periments whi h are interesring, in so far as they serve to 

 manifest a remarkable co-relation between the crystalline 

 forms of the same bodies and the positions of their electri- 

 cal pole?, fjnt these c\pcrnnenis are at the same time de- 

 licate, particularly when we employ cr\stals of borateil 

 magnesia, which, with a volume scarcely exceeding two or 

 three millimetres, have eitrht poles opnosite to each other, 

 in pairs; the powers of which have little enerjjv, and reside 

 each in a single point. Freviouslvto the publication of my 

 Treatise on Mmeralosv, I was occupied with the construc- 

 tion of an apparatus which should be at the same time con- 

 venient, and sufficiently sensible to leave no doubt as to the 

 results of the experiments in question. That which I have 

 described in my Treatise, tome i. and in which I afterwards 

 made a change as subsequently described in a former volume 

 of these Annals, would appear to be perfect in every respect, 

 if its effects were not subject, like those of all electrical 

 machines, to be affected by the state of the atmosphere. I 

 shall briefly mention here, that tins apparatus consists of a 

 small needle (PI. HI. fig. 1.) c<f copper or silver, terminated 

 bv two globules, moveable on a pivot, and isolated, to which 

 we give at pleasure the vitreous or resinous electricity, bv 

 the action exercised upon it by an idio-electrical body to 

 which friction has communicated the contrary electricity. 

 If ihis bodv is a slick of wax, for instance, we (ireseul it at 

 a few centimetres flistance from the stalk which supports 

 the small needle, at the same time that we keep a finger 

 applied to the toot a of this stalk : we after^vards remove 

 the finger, then the stick of wax, and in this ca«e the ap- 

 paratus i? vitreously electrified. Now, when the air is 

 loaded with aqu.'ous vapours, its influence on the metallic 

 needle destroys in an instant the electrical virtues of the 

 latter, or rend^-rs it so feeble and fuoacious, that the opera- 

 tor h compelled to abandon the experiment, and to wait 

 for dry weather. In a public leciurc-rnom, a numerous 

 audience produces a smiilar effect to that of dan)puess. I 

 hdve on such occasions tried in vain to electril'y by fricliou 



* /Inn. (Ill Mitsaim d'His/oire NulurdU, tome iv p. I. 

 Vol. 38. No. ifiO. Atn'ust 1811. F a stick 



