1877.] 



Contact of different Substances. 



301 



then be connected with the respective terminals of the electrometer, the 

 whole being well insulated. By the same key the alternate pairs of qua- 

 drants, in connexion with the respective plates of the condenser, can be 

 charged to a definite difference of potential by a voltaic cell, and can then, 

 while still in connexion with the condenser, be insulated without the least 

 risk of introducing an unknown potential by touching the metallic por- 

 tions of the apparatus by the hand. 



"With this apparatus the following experiments were performed. 



The upper condenser-plate of copper was connected by an insu- 

 lated copper wire with one terminal of the electrometer, the other ter- 

 minal being connected by an insulated copper wire with the lower con- 

 denser-plate, which was again connected with a copper wire dipping in 

 distilled water, contained in a vessel standing on the lower plate. 



The two insulated wires connecting the condenser and electrometer 

 were now brought into metallic connexion by the key, thus putting all 

 four quadrants of the electrometer in connexion, and at the same time 

 uniting the copper condenser-plate with the wire dipping in the water. 

 The copper plate and the water thus become charged, the difference of 

 potential being that due to the contact of copper and water. 



The upper (copper) plate is next brought near the surface of the water, 

 which acts as the lower plate of the condenser, the interval between the 

 surfaces being from 04 millim. to 0*2 millim. 



The metallic connexion between the connecting wires is now sup- 

 pressed, and the upper condenser-plate quickly raised ; the needle of the 

 electrometer is at once deflected, and in the direction which indicates that 

 the upper plate is positive with respect to the lower. 



It appears, then, that when copper and distilled water are in contact 

 they assume a difference of potential, copper being positive to distilled 

 water. 



Excepting Professor W. G. Hankel*, all previous observers, with 

 whose results I am acquainted, agree in asserting that copper is negative 

 to water ; but I have repeated the above experiment a great number of 

 times under varied conditions, and always with the same result, so that 

 I feel confident that copper is really positive to water. 



When the vessel on the lower plate contained a saturated solution of 

 copper sulphate, and similar operations were performed to those above 

 described, the result was that copper and a saturated solution of copper 



* Abhandl. d. konigl. sachs. Gesellschaft der Wissenschaften, vol. xi. p. 605. 



My experiments gave no indication of the change of sign of the potential of copper 

 with respect to that of water, which, according to Professor Hankel, takes place during 

 the first few minutes of contact. It is possible, however, that this change may have 

 taken place, although the copper wire had not been immersed more than two or three 

 minutes when the difference of potential was observed. 



At the time my experiments were performed I had not seen Professor Hankel's 

 paper. 



