CHAPTER I, 



DISCOVERY OF VOLTAIC ELECTRICITY. 



WE have given the name of mcchanico-cliemical to the class of 

 sciences now under our consideration ; for these sciences are con- 

 cerned with cases in which mechanical effects, that is, attractions and 

 repulsions, are produced ; while the conditions under which these 

 effects occur, depend, as we shall hereafter see, on chemical relations. 

 In that branch of these sciences which we have just treated of, Magnet- 

 ism, the mechanical phenomena were obvious, but their connexion with 

 chemical causes was by no means apparent, and, indeed, has not yet 

 come under our notice. 



The subject to which we now proceed, Galvanism, belongs to the 

 same group, but, at first sight, exhibits only the other, the chemical, 

 portion of the features of the class ; for the connexion of galvanic phe- 

 nomena with chemical action was soon made out, but the mechanical 

 effects which accompany them were not examined till the examination 

 was required by a new train of discovery. It is to be observed, that I 

 do not include in the class of mechanical effects the convulsive motions 

 in the limbs of animals which are occasioned by galvanic action ; for 

 these movements are produced, not by attraction and repulsion, but by 

 muscular irritability ; and though they indicate the existence of a 

 peculiar agency, cannot be used to measure its intensity and law. 



The various examples of the class of agents which we here consider, 

 magnetism, electricity, galvanism, electro-magnetism, thermo-electri- 

 city, differ from each other principally in the circumstances by which 

 they are called into action ; and these differences are in reality of a 

 chemical nature, and will have to be considered when we come to treat 

 of the inductive steps by which the general principles of chemical 

 theory are established. In the present part of our task, therefore, we 

 must take for granted the chemical conditions on which the excitation 

 of these various kinds of action depends, and trace the history of the 

 discovery of their mechanical laws only. This rule will much abridge 

 the account we have here to give of the progress of discovery in the 

 provinces to which I have just referred. 



