632 SUPPLEMENTARY. 



649. ELECTRICAL HEATING OF TOURMALINE. Pyroelectrical 

 phenomena give rise to analogous considerations. 



The pyroelectricity of crystals is explained, on Faraday's theory 

 (118), by assuming that the crystal is in a state of electrical 

 polarization, the external effect of which is equivalent to that of two 

 layers, of equal masses and contrary signs, distributed on the surface. 

 When the crystal is at a constant temperature, the surrounding 

 medium, either by its own conductivity, or by the surface of 

 the crystal itself, soon acquires a superficial electrification, which 

 neutralises the former, and annuls its action on any external point. 



When the crystal is broken perpendicularly to the electrical axis, 

 the whole of each of the two fragments then are electrified in 

 opposite directions, not only by the new layers which the polarization 

 on the broken surfaces produces, but also in consequence of the 

 electrification induced on the old surfaces, the equilibrium of which 

 is broken. 



When the temperature changes, the electrification changes 

 also ; but the equilibrium produced by the electrification of the 

 surrounding medium is only set up by degrees, and more or less 

 slowly, according to the conductivity of the medium or of the 

 surface of the crystal. 



If this explanation of pyroelectricity is correct, it follows that a 

 pyroelectrical crystal should be heated or cooled when it is moved 

 in an electrical field, like magnetised iron in a magnetic field. 



Tourmaline becomes heated if it is displaced in such a manner 

 that the influence of the field tends to increase its polarization, and 

 is cooled in the contrary case. 



The effect produced on tourmaline does not depend on the 

 electrification of the surface, and we arrive at this remarkable re- 

 sult; a pyroelectrical crystal which appears in the natural state, its 

 properties being neutralised by the electrification of the medium, 

 undergoes, when it is displaced in a field, the same variations of 

 temperature as if its electrical properties were apparent that is to 

 say, as if it had been raised to a high temperature, then dried, and 

 rapidly cooled. 



650. PRINCIPLE OF THE CONSERVATION OF ELECTRICITY. 

 Whenever a system of bodies, disconnected from any external bodies, 

 is the seat of any electrical phenomenon (8), the total quantity of 

 electricity which it contains remains unchanged. This principle is 

 verified in all experiments, and it is a consequence of Maxwell's 

 views as to the constitution of the medium which serves for 

 conveying electrical force. Although we are not able to affirm that 



