E L E C T R I C I T V. 



425 



UiiUiry. feet conductors of tliis^kind which he found, were some 

 ^ ~,^ ' pieces of pit charcoal, which appeared to be in every 

 Labours of res p ec t -^ perfect conductors as the metals. Dr Priest- 

 ley observed a great variety in the electrical properties 

 of different pieces of pit charcoal ; but he had not as- 

 certained the circumstances in the mode of preparing 

 it, from which this variety was produced. Dr Priest- 

 ley likewise found that common coal cinders from an 

 open fire were little inferior to charcoal in their con- 

 ducting power. This discovery, says a celebrated 

 chemist, * " is a curious and unexpected analogy be- 

 tween the metals and charcoal ; and chemistry furnish- 

 es us with several others no less striking and impoit- 

 ant. When the conducting power of charcoal was tried 

 by succeedin^ electricians, it was found to vary in the 

 most unaccountable manner, sometimes scarcely con- 

 ducting at all, sometimes imperfectly, and sometimes 

 remarkably well. This diversity indicates a difference 

 in the nature of different specimens of charcoal. Hence 

 we may be assured that charcoal is a compound, and 

 that it often varies in the proportion of some one or 

 other of its ingredients." Dr Priestley also found that 

 there was a current of air from the points of bodies 

 electrified plus or minus ; that hot glass is a conductor ; 

 that electricity is easily diffused over the surfaces of 

 new glass tubes, and therefore that the first coat of new 

 glass is in some measure a conductor of electricity, but 

 that after six or seven months these tubes lose this facul- 

 ty, and are as easily excited as older tubes; and that the 

 metallic tinge was much better communicated to the new 

 than to the old glass, twice the quantity of metal being 

 in all cases struck into it, a result which Dr Priestley 

 ascribes to the greater m;ignitude of its pores. Dr 

 Priestley found also that coloured circular spots are pro- 

 duced upon metallic plates, which receive an electrical 

 shock from a strong battery. The most beautiful ap- 

 pearance of this kind was one which he made upon 

 a gold watchcase The central spot was generally 

 formed of shining dots, and the external circle consisted 

 of cavities resembling those of the moon as they appear 

 through a telescope ; but in the spot upon the gold case 

 there appeared, in several places of it, hollow bubbles 

 of the metal. The cavities were deeper in some metals 

 than in others, and he thought that the following was 

 the order, beginning with those in which they were 

 deepest, tin, lead, brass, gold, steel, iron, bismuth, zinc, 

 copper, silver. Dr Priestley likewise observed, that 

 when an electrical battery is discharged, light bodies 

 placed near the electrical circuit are put in motion, an 

 effect which he ascribes to the sudden elasticity given 

 to the air. I le found that a long circuit conducts worse 

 than a short circuit, even when the conductors are the 

 same ; and that when the circuit contains an imperfect 

 conductor, a shock passes to bodies near, and yet no 

 electricity is comrminica't-d. 



The late Mr Cavendish, whose genius has extended 

 so many branches of physics, was likewise a successful 

 cultivator of electricity. He conceived and executed 

 the design of explaining all the phenomena of electri- 

 city by a simple theory, and his paper was actually 

 drawn up and ready for the Royal Society, when he 

 was made acquainted with the theory of JEpinus, of 

 which, though published ten years before, he had never 

 received the slightest notice. The theory of Cavendish 

 may be considered as absolutely the same with that of 



Diieavtrie* 

 nd labours 

 f Caven- 

 riish. 177D. 

 Born 1731. 

 Oitd 1810. 



jEpinus, though it is more accurate and more extended. History- 

 " Since I wrote the following paper," says Mr Caven- ^ 

 dish, " I found that this way of accounting for the 

 phenomena of electricity is not new. jEpinus, in his 

 Tcntamen Theories, &c. has made use of the same, or 

 nearly of the same, hypothesis that I have ; and the con- 

 clusions he draws from it .agree nearly with mine, as 

 far as he goes. However, as I have carried the theory 

 much farther than he has done, and have considered the 

 subject in a different, and, I flatter myself, in a more 

 accurate manner, I hope the Society will not think this 

 paper unworthy of their acceptance." In this paper, 

 Mr Cavendish lays down the hypothesis, and examines 

 by as strict mathematical reasoning as the subject will 

 admit, the various consequences that flow from the hy- 

 pothesis, and then he compares it with the various ex- 

 periments that have been made on the subject. Mr 

 Cavendish informs us, that he proposed soon to publish 

 the results of some experiments in which he had been 

 engaged for the purpose of trying the truth of his hypo- 

 thesis, and of discovering the law of electric attraction 

 and repulsion, i 



Notwithstanding the immense interval which elapsed 

 between the publication of ^pinus' theory and the ap- 

 pearance of Mr Cavendish's paper, yet no person can 

 doubt that the latter is entitled to the credit of a 

 second discoverer. Among his countrymen, at least, he 

 is fully entitled to this honour; and if foreigners, who 

 were less acquainted with the integrity of his character, 

 should hesitate in conferring this secondary honour, we 

 should at least have expected some acknowledgment 

 of his* labours, and some portion of prai-e for having 

 improved and extended the theory of the Russian phi- 

 losopher. We were therefore not a little surprised to 

 find, fhat the name of Cavendish never once appears 

 in the exposition of ^pinus' theory by the Abbe Hauy, 

 published sixteen years after the appearance of Caven- 

 dish's paper, nor in the extract from the Register of 

 the Academy of Sciences, in which Laplace, Cousin, 

 and Legendre, have given an account of the Abbe's 

 work. 



The extraordinary shocks given by the torpedo ha- 

 ving been found by Mr Walsh to be purely electrical,t 

 Mr Cavendish began a series of experiments, with the 

 view of obtaining an explanation of these singular ef- 

 fects. As the shock of the fish was given under water, 

 and was incapable of being transmitted through water, 

 of producing electric light, or of affecting an electro- 

 meter, the difficulty of obtaining a proper explanation 

 seemed to be very great. He constructed an artificial 

 torpedo made of wood, connected with glass tubes and 

 wires, and covered with a piece of sheep's skin leather, 

 and found in numerous experiments, that the effects of 

 this apparatus agreed very well with those of the na- 

 tural torpedo. Sec p. 4~6. 



Mr Cavendish likewise discovered, that iron wire 

 conducts about 400,000,000 times better than rain or 

 distilled water, or in other words, the electric fluid ex- 

 periences no more resistance in passing through a piece 

 of iron wire 400,000,000 inches long than through a 

 column of water of the same diameter only one inch 

 long. He also found that sea- water, or a solution of 

 one part of salt in one of water, conducts 100 times 

 better than fresh water, and a saturated solution of sea- 

 salt 720 times better than fresh water. The former 



vot. vin. PART n. 



* Dr Thomson's Hittory of the Royal Society, p. 4-k 

 t See fkO. !>. 1773, vol. liiii. p. 41. 



