Intelligence and Miscellaneous Articles. 225 



between which the crystal was compressed by means of a solid 

 wooden lever. One of the tin foils being in communication with 

 the earth, the other was connected with the needle of a Thomson- 

 Ma scart electrometer. The deflection obtained after one variation 

 of pressure was proportional to the quantity of electricity liberated 

 — the capacity of the tin foil, under the conditions just now de- 

 scribed, being always negligible in comparison with the capacity of 

 the electrometer. 



Tourmalines which are transparent, colourless or slightly coloured 

 green, yellow, or rose, are in general perfectly insulating ; and such 

 only were employed for the quantitative experiments. Whatever 

 may be their tint, these tourmalines seem to be nearly equivalent 

 in regard to the electrical phenomena ; the differences, if there are 

 any, are certainly very slight ; still it would be necessary to exa- 

 mine a considerable number of specimens before we could affirm 

 that it is always so. 



More or less opaque or black tourmalines are conductors of 

 electricity. A black tourmaline gave the electrometer-needle an 

 impulse equal to about one fifth of the deflection obtained for the 

 same weight with a transparent tourmaline ; moreover the needle 

 returned rapidly to zero. 



The deflections the equality or proportionality of which it was 

 necessary to verify, considering the neglected causes of error, were 

 exact only to within a twentieth part of their value. We have not 

 judged it necessary to attempt to attain a closer approximation ; for 

 the accuracy of the laws enunciated results from the considerable 

 differences between the dimensions of the tourmalines employed. 



For one and the same surface the lengths varied from 0*5 millim. 

 to 15 millims., therefore in the proportion of 1 to 30. For one 

 and the same length the surfaces varied from 2 square millims. to 

 1 square centim., therefore in the proportion of 1 to 50. Given, 

 therefore, the degree of approximation of the experiments, and sup- 

 posing the laws enunciated to be limiting laws, we can certify that 

 when the length is doubled the difference from the true law is less 

 than a six-hundredth, and when the surface is doubled the dif- 

 ference is less than a thousandth. 



A piece measuring 1 cubic millim. sets free, for one and the 

 same pressure, the same amount of electricity as a piece of several 

 centim s. volume. Finally, the effect produced by the addition of 

 one of the first kilograms is sensibly the same as that produced by 

 the hundredth kilogram, for a surface of 1 square centim. 



Gaugain, in a remarkable memoir, has shown the simplicity of 

 the pyroelectrical phenomena of tourmaline. The laws enunciated 

 by him may be placed over against those which form the subject 

 of this Note. It is easy to see that they can be traced one upon 

 the other, if we permit ourselves to be guided by the hypothesis 

 that the phenomena resulting from variations of pressure and those 

 resulting from variations of temperature are due to one and the 

 same cause — namely, the contraction or dilatation in the direction 

 of the axis of the tourmaline. — Comptes Rendus de VAcademie des 

 Sciences, 1881, No. 4, t. xcii. pp. 186-188. 



