194 Scientific Intelligence. — Mineralogy. 



cules undergo when the pressure is diminished, have no influence 

 in modifying the disengagement of electricity. Elasticity is 

 therefore a principal cause of the eifects. The electrical phe- 

 nomena of pressure and those of cleavage have strong relations 

 to each other ; for, when plates of mica or selenite are quickly 

 separated, each of them bears an excess of opposite electricity. 

 If they are brought together again, and placed in the position 

 which they originally occupied, a slight pressure being at the 

 same time applied, the same electrical phenomena are obtained 

 as when they were separated. We therefore see, that pressure, 

 which effects a mechanical approximation of the molecules, 

 produces the same effects as the force of aggregation, which only 

 determines a more immediate contact of the same molecules. 

 These phenomena do not take place indefinitely ; for the ex- 

 posure to the air of newly cleft laminas deprives them pretty 

 rapidly of their electrical propensity, perhaps on account of 

 the hygrometric water which they absorb. All regularly crys- 

 tallized substances possess the same property as mica and sul- 

 phate of lime. I have proved it with respect to Iceland spar, 

 sulphate of barytes, fluate of lime, topaz, &c. It is essential 

 that the crystal be regularly split, for when it is fractured, 

 it manifests no electrical effect. It may, in fact, be easily 

 conceived, that, if the cleavage is not distinct, there may be 

 laminae which assume one electricity, and others a contrary 

 electricity. It then happens that the sum of all these electrici- 

 ties may be nothing, which is most commonly the case. Topaz 

 presents only one direction of cleavage, perpendicular to the 

 axis of the crystal, according to which the distribution of the 

 electricity takes place, when the temperature of this substance 

 is raised to a certain degree. The most natural supposition 

 which suggests itself is, that the laminae being in two different 

 states of electricity at the moment of their separation, may be 

 considered as the elements of a pile. Now, this is not the case, 

 for it would be necessary that the laminae similarly situated with 

 relation to one of the summits of the crystal, should always as- 

 sume the same electricity by cleavage ; which does not hap- 

 pen, as one electricity is sometimes obtained, and sometimes 

 another. Thus the kind of electricity depends upon circum- 

 stances peculiar to the cleavage, and not upon the position of 



