Prof. Thomson on the Mechanical Theory of Electrolysis* 435 



established by Camot and Clausius as the criterion of perfection 

 for a thermo-dynamic engine*, is applicable to this electro-mag- 

 netic engine ; and is satisfied by it when the disc revolves with 

 an angular velocity infinitely nearly equal to 12, since then y, 

 M', and W are each of them proportional to 12— to, whether this 

 quantity be positive or negative ; and consequently if the motion 

 of the disc relatively to a state of rotation with the angular ve- 

 locity O be reversed, all the physical and mechanical agencies 

 are reversed. 



12. From experiments made at Manchester in the year 1845 

 by Mr. Joule, on the quantity of zinc electrolysed from a solu- 

 tion of sulphate of zinc by means of a galvanic current measured 

 by his tangent galvanometer, I have found the electro-chemical 

 equivalent of zinc to be •07.284 f ; and I am informed by him, that 

 from other experiments which he has made, he finds that the 

 entire heat developed by the consumption of a grain of zinc in a 

 DanielPs battery is as much as would raise the temperature of 

 769 grains of water from 0^ to 1° Cent. J Hence, if we wish 

 to apply the preceding investigations to the case in which the 

 electro-chemical apparatus (§ 4) is a single cell of DanielPs bat- 

 tery, we may consider the consumption of a grain of zinc as the 

 unit of the chemical action which takes place, and therefore we 

 have 



€=•07284, (9=769. 



Again, according to Mr. Joule^s last researches on the me- 

 chanical equivalent of heat, the work done by a grain of matter 

 in descending through 1390 feet is capable of raising the tem- 



* " If an engine be sucli that, when it is worked backwards, the physical 

 and mechanical agencies in every part of its motions are all reversed, it 

 produces as much mechanical effect as can be produced by any thermo- 

 d.3Tiamic engine, with the same temperatures of source and refrigerator, 

 from a given quantity of heat." (From § 9 of " Dynamical Theory of Heat." 

 Transactions of the Royal Society of Edinburgh, March 17, 1851, vol. xx. 

 part 2.) 



t See Note on Electro-chemical Equivalents published at the end of this 

 paper. 



X By experiments on the friction of fluids, Mr. Joule has found that the 

 quantity of work necessary to raise the temperature of a pound, or 7000 

 grains, of water from 0° to 1° Cent, is 1390 foot-pounds. Hence the me- 

 chanical equivalent of the consumption of a grain of zinc in Daniell's bat- 

 tery is 152*7, or nearly 153 foot-pounds. Messrs. Scoresby and Joule, in 

 their paper " On the Powers of Electro-magnetism, Steam and Horses," 

 (Phil. Mag., vol. xxvi. 1846, p. 451) use 158 as the number expressing this 

 equivalent according to earlier experiments made by Mr. Joule. The ex- 

 periments from which he deduced the thermal equivalents of chemical 

 action, communicated to me for this paper, are described in a paper com- 

 municated to the French Institute, and acknowledged in the Comptes 

 Rendus for Feb. 9, 1846, but not yet published. 



