300 



On ihe Crystalline Sirw'ture of the 



[April 



ing what Newton calls a "vera causa,"* we have examples of the 

 effects it prodaces as an agent in natural operations ; we may therefore 

 safely reason about its efficiency in this case, and if we are supported by 

 strong analogies derived from its "modus operandi" in other instances, 

 we may be allowed to infer its being the active agent in this, since, to 

 use the words of onef well qualified, to judge, " we are not to deny the 

 existence of a cause, in favour of which we have the unanimous agreement 

 of strong analogies, though in the particular case it may not be apparent 

 how the cause produces its effects." 



These remarks naturally lead us to the more detailed examina- 

 tion of the cause, to which, from a discussion of the assembled 

 group of phenomena and the application of special rules to 

 these, we have been led— and first, we may establish the fact 

 of its existence under circumstances like those of trap dykes, to which 

 we still adhere as the representative of the others. 'l"he discovery that 

 electricity might be excited by the partial application of heat to a circuit, 

 into which no fluid entered as an elementary part, was made as early as 

 1822, by Professor Ssebeech of Berlin.J He employed a single bar of 

 antimony, and having wound round the two extremities of this several 

 coils of brass Avire forming the poles, he applied the heat of a spirit 

 lamp to one end of the bar — immediately a current of electricity w^as 

 established, of intensity sufficient to aifect most sensibly the 

 needle of a galvanometer, placed under the circuit. This discovery 

 of wdiat has been called thermo-electricity (from the cause of its 

 development) excited much interest, and the subject was pursued 

 with great zeal, so that the thermo-electric relations of numer- 

 ous substances were determined— all that seemed essential to the exhi* 

 bition of these, was, that one part of the circuit should have a higher 

 temperature than the other— the immediate consequence of which was, 

 the determination of an electric current in a direction from the hot part 

 towards the cold. There is no necessity for the substances employed 

 being metallic, for M. Nobili§, a Florentine philosopher, has lately pro- 

 duced electric currents, by the contact of two pieces of moist clay, one 



• Professor Whewell in his " History of the Inductive Sciences" questions the 

 right of electricity to be considered a vera causa," on the ground of its not being a 

 physical realitj', but due to the vibrations of an etherial medium. I use the expression 

 without any special reference to the nature of electricity, of which wejknow so little 

 but simply as a means of conveying the desired, expression as to the propriety of rea- 

 soning concerning its operations in the natural world. 



+ Sir J. Herschel. 



% Dr. Roget's Treatise on Electro-Magnetism. 

 \ Mrs. Somerville's Connection of the Sciences. 



