PROGRESS OF ELECTRICAL THEORY. 205 



electrical action the theory was justified. But in order to give it full 

 confirmation, it was to be considered whether any other facts, not im- 

 mediately assumed in the foundation of the theory, Avere explained by 

 it ; a circumstance which, as we have seen, gave the final stamp of 

 truth to the theories of astronomy and optics. Xow AVC appear to 

 have such confirmation, in the effect of points, and in the pheno- 

 mena of the electrical discharge. The theory of neither of these was 

 fully understood by Cavendish, but he made An approach to the true- 

 view of them. If one part of a conducting body be a sphere of 

 small radius, the electric fluid upon the surface of this sphere will, it 

 appears by calculation, be more dense, and tend to escape more ener- 

 getically, in proportion as the radius of the sphere is smaller ; and, 

 therefore, if we consider a point as part of the surface of a sphere of 

 imperceptible radius, it follows from the theory that the effort of the 

 fluid to escape at that place will be enormous ; so that it may easily 

 be supposed to overcome the resisting causes. And the discharge may 

 be explained in nearly the same manner ; for when a conductor is 

 brought nearer and nearer to an electrized body, the opposite electri- 

 city is more and more accumulated by attraction on the side next to 

 the electrized body ; its tension becomes greater by the increase of its 

 quantity and the diminution of the distance, and at last it is too strong 

 to be contained, and leaps out in the form of a spark. 



The light, sound, and mechanical effects produced by the electric 

 discharge, made the electric fluid to be not merely considered as a 

 mathematical hypothesis, useful for reducing phenomena to formula' 

 (as for a long time the magnetic fluid was), but caused it to be at once 

 and universally accepted as a physical reality, of which we learn the 

 existence by the common use of the senses, and of which measures 

 and calculations are only wanted to teach us the laws. 



The applications of the theory of electricity which I have princi- 

 pally considered above, are those which belong to conductors, in which 

 the electric fluid is perfectly moveable, and can take that distribution 

 which the forces require. In non-conducting or electric bodies, the 

 conditions to which the fluid is subject are less easy to determine ; but 

 by supposing that the fluid moves with great difficulty among the par- 

 ticles of such bodies, that nevertheless it may be dislodged and accu- 

 mulated in parts of the surface of such bodies, by friction and other 

 modes of excitement ; and that the earth is an inexhaustible reservoir 

 of electric matter, the principal facts of excitation and the like receive 

 a tolerably satisfactory explanation. 



