of Electriciti) by Pressure. 273 



Let i, i', i", i'", be the electric intensities of Iceland sjiar, 

 of sulphate of barjtes, of hyaline quartz, and of sulphate of 

 lime ; we shall have for the same pressure, i : i': i": i"'\ : 6 : 4, 

 2 : 3, 9 : 1, 9. We see that the electi'ic power in sulphate of lime 

 is about three times less than in Iceland spar ; that is to say, 

 that these two substances, under the same pressure of the elder- 

 pith*, retain an excess of positive electricity, which is three 

 times as strong in the Iceland spar as in the sulphate of lime. 



These experiments show that the electric intensity for 

 pressures of from one to ten kilogrammes is in proportion to 

 the pressure ; that is to say, that for a double pressure the in- 

 tensity is double, and so on ; supposing always that the velo- 

 city of separation is such in each experiment as to give the 

 maximum of electric intensity. 



Does this proportionality extend to much stronger pres- 

 sures, using always the requisite velocities ? This question is 

 very difficult to answer, for the apparatus with which we 

 operate will admit of only very inconsiderable pressures : ne- 

 vertheless, if the development of electi'icity be owing, as appears 

 probable, to the compression of two bodies, that is to say, to 

 the approximation of molecules, we may infer that this deve- 

 lopment must cease when the molecules have attained a certain 

 degree of compression. In fact, the state of the molecules in 

 bodies may be compared to the force of a spring. It follows, 

 then, that the more bodies have been compressed, the less sus- 

 ceptible will they be of fresh compression, and therefore that 

 a point must exist at which the molecules can be brought no 

 closer. If this be really the case, electric intensity ought to 

 increase rapidly at first, and the rate of its increase should 

 diminish slowly. The preceding experiments show, that un- 

 der slight pressures the electric intensities form an increasing 

 geometrical progression. 



Supposing the temjierature to be constant, the electric in- 

 tensity for any pressure /> would have an expression of the 



in which a, A and m would be three constant quantities for 

 the same body, but variable from one body to another ; m 

 should be such that for inconsiderable values of j^ the term 



might be neglected as far as relates to aj). It in Iceland 

 spar naturally split a = 1 "5, we should have 



i = p. 1 '5 + A V p 

 This formula is quite empirical. 



• There is a discrepancy between this statement and that in the prcccd- 

 in;; tables, in which corii, and not eldcr-pit/i, is unilorniiy mentioned as the 

 substance ein|jioyed. — Edit. 



Vol. 62. No. 306. Oct. 1823. M m In 



