262 APPENDIX. [No. XVII. 



perceive that the moment I connect them with a galvanometer, deflec- 

 tion ensues. Animal bodies consist solely of membranes and fluids, and 

 therefore, in the order of my investigations, I had to study batteries, 

 solely composed of similar materials. This form of voltaic circuit is 

 extremely difficult to investigate, though one is placed upon the table for 

 your inspection. 



After I had thoroughly studied the electro-voltaic test, the time 

 arrived to ascertain whether a voltaic current was actually passing during 

 nervous action : for although the analogies which I have detailed were, to 

 my mind, complete, yet analogy would be useless without the corrobora- 

 tion of direct experiment. My first experiment was to introduce two steel 

 needles into a rabbit : the first into the masseter, or muscle which enables 

 the creature to masticate ; the second, into the subcutaneous cellular tissue. 

 After two or three minutes, the creature, which was very tame, attempted to 

 bite my finger ; the power of volition was sent to the muscle : this acted 

 upon my electro-voltaic test, and you may judge of my inexpressible 

 delight when the deflection of the needle showed to my mind the mecha- 

 nism of volition. These needles being between the skin and muscle, the 

 course of the voltaic circuit is clearly demonstrated to exist between these 

 two points, and therefore each required a most minute consideration. 



Sensations are received by various organs which are destined to be 

 acted upon by certain physical forces, as the eye by light, the ear by 

 sound, the nose by odours, the tongue by savours, or the skin by heat 

 or force. 



It is quite certain that if a voltaic circuit is generated in the eye, 

 there must be such contrivances as photo-voltaic circuits ; that is, voltaic 

 circuits in which light causes the evolution of electricity. In trying the 

 experiment, I found that there was not only an extensive series of com- 

 binations in which the sun's rays determine the generation of electricity, 

 but that in one division light caused a positive voltaic circuit; in the 

 second, a negative voltaic circuit. The table of these circuits will illustrate 

 the manner in which these circuits are formed, by using solutions so 

 arranged that one portion may be screened from the light, and the second 

 may be acted upon powerfully by the sun's rays. 



NEGATIVE PHOTO- YOLTAIC CIRCUITS. 



Mixed solutions of proto-sulphate of iron and nitrate of silver. 

 ,j gallic acid and nitrate of silver. 



oxalic acid and chloride of gold. 



ferrocyanate of potash and ammonio-percitrate of iron. 



ferrocyanate of potash and ammonio-pertartrate of iron. 



ferrocyanate of potash and potassio-tartrate of iron. 



POSITIVE PHOTO-YOLTAIC CIRCUITS. 



Mixed solutions of pernitrate of iron and red ferrocyanate of potash. 

 bromine water, phosphorus water, and pernitrate of iron. 



These experiments I cannot show you this evening, because I cannot 

 command the sun's rays to shine upon one side of my apparatus ; but 

 from what I have stated, you will perceive that it is quite within the range 

 of ordinary physical effects to have voltaic circuits set in action by light. 



