No. XVnj APPENDIX. 261 



contrivances, we find tliat similar conditions are fulfilled by a double 

 voltaic circuit. 



Z S 



S z 



If we abstract the proper exciting fluid from either end, or substitute 

 any other fluid, or destroy the structure at onfe end or the other, or divide 

 the connecting portions or wires, the eficcts proper to the apparatus ■will 

 not be manifested, and the battery will be destroyed. The analogy between 

 the mechanism of a double voltaic circuit and that of animal life is 

 quite complete ; for if we pith an animal, an operation which separates the 

 brain from the body, or remove the blood from the brain or from the 

 peripheral part, or destroy the stnicture of either the brain or the peri- 

 phery, action is stopped, and animal life ceases. 



Tou wlU at once say, doubtless, that man has no metallic wires, no 

 plates ; and therefore, you may naturally ask, how far does that fact 

 destroy the analogy which I have given to you ? It is not necessary, 

 however, that the connecting portion should consist of metal ; and though 

 all present are doubtless accustomed to see the electric telegraphic wires 

 along the course of the railways, yet I have here upon the table an example 

 of fluid telegraphic conductors, which answer as efBciently for the con- 

 ducting of the voltaic force, as wires or metals. Those amongst you who 

 reside at Upper Clapton, may remember some time since to have seen 

 mysterious wii-es placed at an elevated situation round the Horse-shoe 

 . Point on the river Lea. At the time these wires were in that situation, I 

 was experimenting upon_ the conducting power of liquids, and they were 

 found to possess that property in an extraordinary degree. If the nerves, 

 however, carry the voltaic force, they might perhaps be expected to have 

 within themselves some means of insulation; and from my ovm micro- 

 scopical examination of nei-ve-fibre perfectly fresh, I believe that a layer 

 of fat exists in the interior of each primitive fibril, which would as effi- 

 ciently insulate it as the gutta-percha of my tube does these ai-tificial 

 nerves which are placed on the table. 



In this double voltaic apparatus before you, in which the communi- 

 cating portion consists of gutta-percha tubing, filled vrith acid and water, a 

 powerfiil voltaic current is passing' but one which will yield no indications 

 of its presence to ordinary voltaic tests. It is no easy matter, gentlemen, 

 to prove the presence of a voltaic current in a fluid, and for a long period 

 I did not know how to proceed to render its existence certain. However, 

 at last I observed, if any metal capable of being oxidized was interposed in 

 the path of a voltaic circuit, that one portion becomes positive, the other 

 negative : and that this result is no fanciful chimera, I now show you an 

 electro-metallurgic precipitating trough, in which a piece of copper is 

 inserted between the positive and negative plates, and you will at once 

 perceive that the poi-tion neai- the negative pole has become acted upon 

 or positive, the part neai-est the positive pole has become negative, and 

 has metallic copper deposited upon it. Prom this experiment I saw 

 that a mode was afforded to me of ascertaining the presence of a voltaic 

 circuit in any fluid. To give you a practical illustration of the value 

 of the electro-voltaic test, I have introduced two copper wires into one 

 of the gutta-percha tubes constituting my artificial nerves, and you will 



