ELECTRICITY. 



the admirable hypothesis of negative and positive elec- 

 Electriinty. tricity advanced by Dr Franklin, which afterwards 

 S """V ^ formed tlic ground work of the more perfect theories of 

 jEpinus and Cavendish. 



To enter into any farther details upon these hypothe- 

 ses, would be to waste the time without adding to the 

 information of the reader. 



Froteoor The late Professor Russel, who filled the chair of 



hy- \atural Philosophy in the University of Edinburgh 

 'poi!.t:i). w i tn distinguished ability, endeavoured to explain the 

 phenomena of electricity, by a substance which he called 

 the electrical fluid, which was connected with bodies 

 by attractive and repulsive forces, diminishing as the 

 distance increases. He considered the electrical fluid 

 as a compound fluid, containing elementary fire, and 

 deriving from it a high degree of elasticity, or a mu- 

 tual repulsion of its particle. This repulsive force, 

 however, acts at a distance ; and hence bodies contain- 

 ing more electric fluid than the spaces around them, re- 

 pel each other. The principal ingredient of the com- 

 pound fluid is electricity, which is united wfththe elastic 

 fluid by chemical affinity or elective attraction. This at- 

 traction extends to all distances, but diminishes accord- 

 ing to a law different from that of the mutual repulsion of 

 the elastic fluid ; and its general tendency is to repress 

 these repulsions when the fluid exists in its compound 

 state. Conducting bodies are attracted by the elect yi<-ltij 

 at all distances, but it attracts electrics only nt insensible 

 distances, and at such distances its own particles attract 

 each other. This complex theory, in which the forces 

 areall accommodated to the phenomena, though proposed 

 merely as a conjecture by its ingenious author, was re- 

 ceived at the time with great satisfaction, and accorded 

 pretty well with the common electrical phcnomeirt. 

 l,uc' A hypothesis somewhat similar to that of Professor 

 iypotlu-.-is. Rupsel, w . ls proposed by a celebrated and able natural- 

 M. de I. uc. F laving had occasion to read Volf.Vs 

 theory of electric influences, De Luc was struck with 

 the resemblance between the phenomena of the elcctro- 

 phorus, and the hygroscopic phenomena of the conden- 

 sation and evaporation of moisture; and by a more ex- 

 tensive examination, he observed a striking analogy be- 

 :i all the other phenomena of electricity mid hv- 

 grometry. Me was therefore led to refer all the phe- 

 xnn of electric-it}' to the operation of n compound 

 expansive substnnce, called the electric fluid. This 

 fluid he supposed to consist first of electric mn/lrr, whirh 

 is the gr.-ivitating part of the compound, and ci' the < /<v- 

 iric (IfJ'ercnl fluid, or carrying fluid, b/ which the elec- 

 tric matter was conveyed from one body to another. 

 U'c cannot occupy the attention of the reader with any 

 iarther account of this theory. Those who think it 

 worth their while to pursue it farther, will find it fully 

 letiiled in lie Luc's 7<A : e< xnr Metenrnlogie, if 

 <>r a crenenil view of it in Dr Robison's \y*tem of Me- 

 rhanicnl Philosophy, vol. iv. where an account of Pro- 

 fessor Russet's hypothesis will also be found. See al- 

 xicholson's Journnl, vol. xxviii. p. 3, for some rc- 

 observations of De Luc respecting his own theory. 



:oiy of 



CHAP. II. 



Theory of sEpinus and Cacendish. 

 THE circumstances under which the theories of .Tlpi- 

 nus **"* Caveru " sh wcrc laid before the public, have al- 



ready been sufficiently detailed in our History of Elec- Theoretical 

 tricity. The two theories, in which all the phenome- Electricity 

 na are explained by the agency of a single fluid, are * "" ~^~"" 

 essentially the same, and were framed without any com- 

 munication between these two philosophers. .Spin us, 

 however, having published his theory about ten years be- 

 fore that of Cavendish appeared, lias justly received the 

 credit of the invention ; and even in this country, hi.-* 

 name has been alone associated with the theory which he 

 formed. The justice of tliis decision we are neither 

 disposed nor entitled to question ; but the claims of Mr 

 Cavendish, as a second inventor, have scarcely in his 

 own country received due attention, while on the con- 

 tinent, his name is almost never mentioned by thof 

 illustrious men, who have cultivated and extended the 

 science of electricity since the publication of his valua- 

 ble researches. 



We trust, therefore, that we are doing an net of jus- 

 tice to the memory of that distinguished philosopher, 

 as well as a service to science, in giving, almost in his 

 own words, a full account of his theory, which has ne- 

 ver yet appeared in any work but the Philosophical 

 Transactions, in which it was at first published. The 

 hypothesis of yEpinus has already been more than once 

 published in our own language ; and such of our read- 

 ers as wish to see a full and able abridgment of it, are 

 referred to the fourth volume of Dr Robison's Systcnt 

 of Mechanical Philosophy. 



SECT. I. Cavendish's Hypothesis. 



THE electric jluid is a substance, the particles of which Cavetidich'* 

 repel each other, and attract the particles of all other h >'P 01 ' 1 

 matter, with a force inversely as the square of the dis- 

 tance. 



The' particles of all other matter also repel each other, 

 and attract those of the electric fluid, with a force vary- 

 ing according to the same law. Or, if we consider the 

 electric fluid as matter different from all other matter, 

 the particles of all matter, both those of the electric 

 fluid, and of other matter, repel particles of the sanu- 

 kind, and attract those of a contrary kind, with a force 

 inversely as the square of the distance. * Mr Cavendish 

 never comprehends the electric fluid under the general 

 term matter, but considers it only as another sort of 

 matter. He regards it as indifferent whether all sorts 

 of matter arc supposed to possess in an equal degree 

 the attraction and repulsion described in the hypothe- 

 itT whether some sorts only are supposed to possess 

 it in a greater degree than others. It i.s probable, how- 

 ever, that the electric fluid possesses this property in a 

 much greater degree than any other matter ; for its 

 weight in any body probably bears a very small pro- 

 portion to the weight of the matter in the body, but yet 

 the force with which the electric fluid in any body at- 

 tracts any particle of matter in that body, must be equal 

 to the force with which the matter of the body repels 

 that particle, otherwise the body would appear electri- 

 cal, as will afterwards appear. 



In order to explain this hypothesis more fully, Mr 

 Cavendish supposes that one grain of electric fluid at- 

 tracts a particle of matter at a given distance, with as 

 much force as four grains, (or any other number,) of 

 any matter, such as lead, for example, repel it ; then one 

 grain of electric fluid will repel a particle of electric 



* In Mr Cavendish's own statement of his hypothesis, he characterises the electric force ai a force varying, inversely, as some less 

 power of the distance than the cube ; but as the real law of action b been ascertained, (see page 4W,) we have subitituUd it in the *e. 

 Yd.. VIII. FAUT H. 3 X 



