368 Miscellaneous Intelligence. 



will be either doubled when the double refraction is considerable, 

 or rendered broader or otherwise modified in point of colour, 

 when the double refraction is small. The effects may be cu- 

 riously varied by crystallizing upon the same plate of glass 

 crystals of a decided colour, by which means we should have 

 white and coloured haloes succeeding each other. — Edin. 

 Phil. Jour. viii. 394. 



4. On the Electricity produced by Pressure.— A very import- 

 ant paper, on the developement of electricity by pressure, and 

 the laws of that developement, by M. Becquerel, is to be found 

 in the Annales de Chimie, xxii. 5. We cannot do more at pre- 

 sent than translate the summary given at the conclusion of the 

 paper, 



1 1 is seen, then, that all bodies assume two different electric states 

 by pressure: that, in two bodies being perfect conductors, this 

 state of equilibrium ceases, at the moment the pressure is re- 

 moved, but if one be a bad conductor, the effect of the pressure 

 continues for a longer or shorter time.: that the pressure alone 

 maintains the equilibrium of the two fluids, placed on each of 

 the surfaces ; for if the pressure be diminished, and, at the end 

 of a certain time, the bodies be removed from the compression, 

 they will be found to have the electricity, due only to the last 

 or remaining pressure : that heat modifies the developement of 

 electricity in a particular manner : that the intensity of the elec- 

 tricity increases, at first, directly as the pressure ; and that it is 

 probable this proportion diminishes at high pressures, as the 

 bodies lose their power of being compressed : finally, it is ren- 

 dered probable, that the light which is disengaged in powerful 

 concussions, is due to the rapid recombination of the two elec- 

 tric fluids developed on the surfaces at the moment of com- 

 pression. 



5. Light evolved by Pressure. — We extract the following 

 passage from the paper above referred to. Considering the 

 increased developement of electricity in bodies, by the augmen- 

 tation of pressure, ought we not to refer to this cause certain 

 luminous phenomena, of which the origin is as yet unknown? 

 For instance, it is said, that in the Polar Seas, it frequently 

 happens, that the blocks of ice which strike together evolve 

 light. These enormous blocks arriving one against the other, 

 with considerable motion, will be submitted to great pressure, 

 and thus the two blocks be placed in two different electric states. 

 At the moment the compression ceases, the two fluids will re- 

 combine, in consequence of the conducting power of the ice ; 

 and may not the light disengaged be the result of the combi- 

 nation of the electric fluids *? 



* See also the light from the falling of a glacier, ix. p. 426. 



