CHAP, ix.] NERVOUS AND ELECTRICAL FORCES COMPARED. 237 



afford many examples of men the working of whose minds has been 

 wholly or partially destroyed by the shock which a sudden reverse 

 of fortune, or the loss of some near and dear relative, may have 

 occasioned. Constant or frequent excesses in the use of ardent spirits 

 may probably be thus injurious in two ways ; first,, by the direct 

 influence of the alcohol on the cerebral fibre itself, producing a chem- 

 ical alteration in the nervous substance; and secondly, by the fre- 

 quent mental excitement which the use of such a stimulus induces. 



Can we form any conception of the nature of this wonderful 

 power, which is so intimately connected with the functions of our 

 bodies and with the working of our minds ? That it presents many 

 points of resemblance to electricity, a comparison of the laws of 

 these two forces leaves no room to doubt ; although there are 

 abundant reasons for questioning their identity. The comparison 

 may be best instituted between the nervous power and the force 

 of voltaic electricity, or current affinity, as it has lately been called, 

 which is developed in the galvanic battery. For the production of 

 this force the ordinary requisites are two dissimilar metals, and an 

 interposed compound liquid. When the metals are brought into 

 contact with each other, a chemical action immediately commences, 

 and an electric current sets in a definite direction, namely, from the 

 metal which exerts the greatest affinity for one of the elements of 

 the interposed liquid towards the other metal. Thus if zinc, pla- 

 tinum, and dilute sulphuric acid be used, the fluid is decomposed ; 

 its oxygen is attracted to the zinc, which being oxidised and uniting 

 with sulphuric acid, sulphate of zinc is rapidly formed, and dissolved 

 as quickly as formed, in the liquid ; its hydrogen is evolved at 

 the platinum. So long as there is fluid for decomposition, and so 

 long as contact between the metals is maintained, these phenomena 

 will continue. During the continuance of these chemical actions, 

 the metals as well as the interposed fluid are supposed to be in a 

 peculiar molecular condition, upon which the development of force 

 in a current form depends. Commencing from the immersed portion 

 of the zinc, each particle, whether of metal or fluid, communicates its 

 peculiar state to that which succeeds it, until the whole circuit, from 

 the zinc through the fluid to the platinum, and back again to the 

 zinc, is in the same state, one, namely, of polarity or electrical ten- 

 sion. A similar state may be induced in glass, sealing-wax, &c. 

 by friction ; or in two dissimilar metals in intimate contact, by heat- 

 ing them at the place of junction ; or in one metal, as a coil of 

 platinum wire, by heating it unequally (thermo-electricity) . The 

 simple contact, indeed, of two plates of different metals with 



