5J^ M. Becquerel on the electrical properties of Tourmaline. 



the laws of symmetry in the configuration of their summits are 

 electrical by heat. 



I have thus, I presume, given a summary of all that has been 

 observed respecting the properties of the tourmaline down to 

 the present time. 



I commenced my inquiries by observing what took place in 

 a tourmaline, 1^^, when all its parts were equally heated or 

 cooled at the same time: Sid, when one of the sides received 

 more heat than the other, whether this heat was increasing or 

 decreasing. I first suspended the stone in paper by a single 

 silk fibre, which descends into a glass vessel placed in a basin of 

 iron filled with mercury, and the temperature was then raised 

 by a spirit-lamp, upon which it was placed. In proportion as 

 the interior of the vessel became heated, the temperature of the 

 tourmaline was raised, and, as it was very easily set in motion 

 from its mode of suspension, the slightest signs of electricity 

 were quickly observed. A thermometer placed at a short dis- 

 tance from the tourmaline indicated its temperature. With 

 this apparatus I obtained the following results : At 30° Centi- 

 grade, the electrical polarity began to be sensible at the ap- 

 proach of a feebly electrified body, and it continued as far as 150°, 

 and beyond it, provided the temperature continued to rise; for if 

 it was stationary an instant the polarity disappeared immediate- 

 ly, so that there was no appearance of electricity as long as the 

 temperature was constant ; but the moment it diminished, the 

 polarity reappeared, but of an opposite character. The pole 

 which was originally positive became negative, and vice versa. 

 These effects always took place at whatever time the elevation 

 of the temperature was arrested. The time of the passage from 

 one polarity to another was very short. 



From this one would think, that the electrical intensity of 

 each pole was proportioned to the quickness of the heating or 

 of the cooling ; but it does not appear to be so. In order to 

 observe what will happen, it is necessary to measure tlie elec- 

 trical intensity at any epoch whatever. This is done by placing in 

 the interior of the vessel of glass containing the tourmaline, and 

 at a little distance from each of its extremities, two vertical rods 

 of iron, each communicating with one of the poles of a dry 

 pile^ whose electrical intensity may be considered as constant, 



