188 APPENDIX TO MEMOIR OF PELTIER. 



proportional to theii- length, and inversely proportional to the surface of their 

 transverse section. It Avoukl be desirable, certainly, that this law might be 

 considered exact ; but unfortunately it is not so, for Peltier has demonstrated 

 that there are very great dijSerences between the losses undergone by a current 

 whicli traverses different lengths of the same wire according to the kind of elec- 

 tro-motor employed ; according as we have to do with a hydro-electric pile, a 

 thermo-electric pile, or with electricity by induction. Further, for the same 

 electro-motor the results vary according to the greater or less power of the dis- 

 turbing action.* , 



IiC2>ly of Peltier to an objection made to the chemical theory of the voltaic j^Hc. — 

 The partisans of the theorj^ of contact had often objected that it is not always 

 the bodies most strongly attacked which give most electricity. How, said they, 

 should chemical action be the cause of currents, when we obtain by the least 

 oxidation of zinc in pure water a current superior to that given by copper 

 plunged in nitric acid, which devours it in a few instants ? Peltier has supplied 

 the explanation of this apparent anomaly, t 



To have full fnombreuxj currents, it is not only necessary that there should- 

 be much electricity produced, but, moreover, that the two electricities should be 

 collected, each separately, at the moment of their production ; this takes place 

 with the zinc, but does not take place with the copper. When an acid attacks 

 and oxidizes the zinc, that oxide remains adherent to the metallic plate ; the 

 negative electricity can therefore easily diffuse itself over this last. On the 

 contrary, when the acid attacks copper, the resulting oxide does not remain 

 adherent to the metal ; it falls into the acidulated liquid, leaving the copper still 

 bright and clean. Of course, in this case, there must be a vast quantity of 

 electricity lost ; in eff"ect, the chemical combination is no longer accomplished, 

 as in the preceding case, in contact with a good conductor ; it takes place in the 

 midst of an acidulated liquid ; it hence results that the negative electricity is 

 recombined, in part at least, with the positive electricity which is present in the acid. 



What has been said abijve explains the utility of the amalgamation of the 

 positive elements in batteries ; the combination of the oxygen of the solution 

 not being capable of accomplishment except in the interstices of the mercury, 

 the electric phenomenon is enveloped by a conducting metal, and the resinous 

 electricity, thus collected from all parts, is propagated through the conductor to 

 become again neutralized with the vitreous electricity abandoned to the liquid. 



IV. — Oyaxo-polarimetrt. 



Peltier had occupied himself much with that branch of the physical sciences 

 which treats of light. It will be readily understood that, desiring to penetrate 

 as far as possible into a knowledge of the intimate structure of bodies, he would 

 not neglect the study of optics. There is, indeed, no science more useful or 

 necessary in this point of view, for there is none in which molecular actions and 

 influences are more distinctly defined; there is none of which the general theory 

 is so complete and satisfying. Independently of many other circumstances, this 

 is referable to a fact which has not perhaps been sufiicientl}' remarked. For the 

 study of caloric, of electricity, of magnetism, there is always need of instru- 

 ments, and these instruments, products of our industry and ingenuity, are always 

 more or less awkward; we must have recourse to the thermometer, the galvan- 

 ometei", the different compasses of declination, inclination, &c. For light, on 

 the contrary, we need them not; the instrument has been conferred on us ready 

 made by nature, and is of an admirable sensibility: it is the eye. Peltier had 

 given, therefore, much attention to the phenomena of light and had perfectly 



" Peltier : Communication to the Academy of Sciences on electric conductibility. Comptes 

 rendus, t. 1, pp. "203, 1835. 



t See Comptes raidus of the Academy of Sciences of Paris, 1837, t. 4, p. G5, and the 

 Dictionnaire Univers. d'Histoire JSaturelle, article Galvanism. 



