Chemical Science, 479- 



aflFects the derelopement. The instrument he used was a balance of 

 torsion, the wire being of plalina, ex<ieedingly fine, and drawn by 

 Wollaston's method. By the aid of this instrument it was ascer- 

 tained, that when badly conducting substances were experimented 

 with, and a convenient velocity of separation obtained, the inten- 

 sity of the electricity disengaged was proportional to the pres- 

 sure, I. e. a double pressQre gave a double intensity; the law, 

 however, diminishes no doubt as the pressure is increased, either 

 as the molecules lose their faculty of being compressed, or as the 

 surfaces of contact alter. By submitting ditferent mineral sub- 

 Stances to the pressure of the same body, their electric relation 

 has also been ascertained, and in this way it was found that Ice- 

 land spar had thrice the power of sulphate of lime. 



When two bodies are pressed together, and the pressure is then 

 diminished without the contact being changed, the electric effect 

 remains some time after the partial removal of the pressure, ac- 

 cording to the conducting power of the bodies ; so that they may 

 be removed from each other, and yet retain an electricity greater 

 than that due to the last pressure. A disc of cork was pressed 

 against a crystal of Iceland spar with a force of 4 kilogrammes j 

 the pressure was then reduced to two kilogrammes, and a minute 

 after the pieces were separated : the electricity of the cork was then 

 170 ; that produced by the original pressure would have been 250 i 

 so that it is sufficiently evident the effect of the greater pressure 

 had remained some time after the pressure itself was lessened. 

 If, on repeating the experiment, in place of separating the two 

 pieces, the pressure is restored to 4 kilogrammes, then dimi- 

 nished, and again re'stored several times, the disc of cork will 

 ultimately be found to have an electricity of 250, or no more than 

 that due to the highest pressure given. 



Hence it results, that if two bodies, one a bad conductor of 

 electricity, are pressed against each other with a certain force, and 

 the force be diminished and increased several times without any 

 change in the place of contact, each of these bodies, when freed 

 from the compression, will exhibit only the quantity of electricity 

 due to the highest pressure. The effect is to complete in each 

 body that quantity of electricity which it should receive in pro- 

 portion to the highest pressure. 



Electricity due to Cleavage. — Different bodies, which adhere 

 together, exhibit electricity upon their surfaces when separated, 

 the two bodies being in opposite states. Glass and gum-lac 

 produce this effect with mercury, and gum-lac with glass. In the 

 phenomena of compression also, when adhesion takes place, as 

 between two pieces of cork, the electricity is greater than when 

 there is no adhesion. These effects are eminently distinguishable 

 when cork or elder-pith is pressed against the polished faces of a 

 diamond. 



The electrical phenomena due to pressure and those of cleavage 



212 



