Professor Faraday's Discoveries. 293 



composable bv the electric current are called electrolytes, and when 

 eleetro-ehemically decomposed, they are said to be electrohjzed ; the 

 substances themselves, which are evolved in such cases, being called 

 zetodes, and the terms zeteisode and zetexode being applied, accordingly 

 as the substance passes in one direction or the other. The propriety 

 and the advantage of employing these new terms, the author observes, 

 can be properly appreciated only by an experience of their uses and 

 applications in the exposition of the theory of decomposition given 

 in the fifth series of these inquiries, and of that of definite electro- 

 chemical action advanced and supported in the present paper. 



The first section of this paper is occupied with the consideration of 

 some general conditions of electro-chemical decomposition. It has 

 been remarked, that elements which are strongly opposed to each 

 other in their chemical affinities are those most readily separated by 

 the voltaic pile ; and the discovery of the law of conduction, explained 

 in the fourth series *, has led to a great augmentation of the number 

 of instances which are in conformity with this general observation : 

 but it is here shown, that the proportion in which the elements of a 

 body combine has great influence on the electro-chemical character 

 of the resulting substance ; and that numerous instances occur where, 

 although one particular compound of two substances is decomposable, 

 another is not. 1c appears, that whenever binary compounds of simple 

 bodies are thus related to one another, it is the proto-compounds, or 

 those containing single proportions, which are decomposable, and that 

 the per-compounds are not so. 



The second section contains an account of a new instrument demised 

 by the author, for exactly measuring electric currents, and which he 

 terms the volta-electrometer. The current to be measured is made 

 to pass through water acidulated by sulphuric acid, and the gases 

 evolved by its decomposition are collected and measured, thereby 

 giving at once an expression of the quantity of electricity which has 

 passed. The principle on which this conclusion is founded is the 

 new law discovered bv the author, " that the decomposing action of 

 any current of electricity is constant for a constant quantity of electri- 

 city." The accuracy of this law was put to the test in every possible 

 way, with regard to the decomposition of water, by making the same, 

 current pass in succession through two or more portions of water, 

 under verv different circumstances: but whatever were the variations 

 made, whether by altering the size of the poles or electrodes, by in- 

 creasing or lessening the intensity of the current or the strength of 

 the solution, by varying its temperature or the mutual distance between 

 the poles, or by introducing any other change in the circumstances 

 of the experiment, still the effect was found to be the same ; and a 

 given quantity of electricity, whether passed in one or in many por- 

 tions, invariably decomposed the same quantity of water. No doubt, 

 therefore, remains as to the truth of the principle on which the volta- 

 electrometer acts : but with regard to the practical application of the 

 principle, (several forms of the instrument are described by the author, 

 ami the mode of employing them, either as the measurers ol absolute 

 quantities, or as standards of comparison, are fully pointed out. 

 ' See Load, and Kdinb. Phil. Mag. vol. iii. pp. 44'J, 450. 



