On Electrolysis and the Passage of Electricity through Liquids. 261 



temperature will naturally present themselves; for example, 

 Foucault^s experiment of the heating of a copper disk when ro- 

 tating in the magnetic field, can be exhibited to a large audience 

 by painting the disk with this iodide; on the disk attaining 

 70° C, the brilliant scarlet will change to a deep brown_, to re- 

 gain its former brilliant hue on cooling. 



xi more useful application may be made of this, or of several 

 other more appropriate metallic compounds, by painting them on 

 ih.Q pillow-blocks and other parts of machines which are liable to 

 injurious heating from friction. Thus the machinist can, from 

 the colours of these paints, ascertain the temperature of these 

 sometimes inaccessible parts of moving machines. 

 May 20, 18/2. 



XXXIII. On Electrolysis, and the Passage of Electricity through 

 Liquids. By G. Quincke. 

 [Conchided from vol. xliii. p. 525.] 

 §57. 



I^HE distribution of free electricity in a closed circuit is known 

 especially from Kohlrausch^s experiments*, wdio connected 

 a linear DanielFs battery by a zigzag-shaped thin metal wire, and 

 connected two different places of the circuit with the plates of a 

 condenser. The electricity which one condensing plate assumed 

 when the other was connected with the earth was determined by 

 means of a Dellmann's electrometer, and was found, in accord- 

 ance with theory, to be proportional to the difference of poten- 

 tials (tensions) of the free electricity at the two places of the 

 circuit. It was here quite immaterial which point of the same 

 section of the circuit was in conducting communication with the 

 condensers, and also how great was the resistance of the con- 

 necting wires. 



Experim.ents were indeed made by Ermanf on the free electri- 

 city of a column of water when traversed by an electrical current ; 

 but on the one hand the apparatus in those days were by no 

 means so perfect as at present ; and then I wanted to ascertain 

 experimentally that distilled water, an extremely bad conductor 

 of electricity, indicates free electricity on the passage of an elec- 

 trical current through it, just as do metals or solution of sulphate 

 of copper. 



Eor this purpose the above-described (§ 56) glass trough, after 

 the wires had been removed, was filled with pure distilled water. 

 In thiswere immersed two rectangular platinum plates 60 millims. 



* Po^g. Ann. vol. Ixxviii. p. 1 (1819). 



t Gilbert's xinn. voh viii. (1801) p. 20/, and vol. x. (1802) p. 11. 



