Determination of Electro-chemical Equivalents. 427 



835. x. Electro-chemical equivalents are always consistent, 

 i. e. the same number which represents the equivalent of a sub- 

 stance A when it is separating from a substance B, will also 

 represent A when separating from a third substance C. 

 Thus, 8 is the electro-chemical equivalent of oxygen, whether 

 separating from hydrogen, or tin, or lead; and 103*5 is the 

 electro-chemical equivalent of lead, whether separating from 

 oxygen, or chlorine, or iodine. 



836. xi. Electro-chemical equivalents coincide, and are the 

 same, with ordinary chemical equivalents. 



837. By means of experiment and the preceding proposi- 

 tions, a knowledge of ions and their electro-chemical equiva- 

 lents may be obtained in various ways. 



838. In the first place, they may be determined directly, as 

 has been done with hydrogen, oxygen, lead, and tin, in the 

 numerous experiments already quoted. 



839. In the next place, from propositions ii. and iii. may 

 be deduced the knowledge of many other ions, and also their 

 equivalents. When chloride of lead was decomposed, platina 

 being used for both electrodes (395.), there could remain no 

 more doubt that chlorine was passing to the anode, although 

 it combined with the platina there, than when the positive 

 electrode, being of plumbago (794.), allowed its evolution in 

 the free state ; neither could there, in either case, remain any 

 doubt, that for every 103*5 parts of lead evolved at the 

 cathode, 36 parts of chlorine were evolved at the anode, for 

 the remaining chloride of lead was unchanged. So also when 

 in a metallic solution one volume of oxygen, or a secondary 

 compound containing that proportion, appeared at the anode, 

 no doubt could arise that hydrogen, equivalent to two vo- 

 lumes, had been determined to the cathode, although, by a' 

 secondary action, it had been employed in reducing oxides of 

 lead, copper, or other metals, to the metallic state. In this 

 manner, then, we learn from the experiments already de- 

 scribed in these Researches, that chlorine, iodine, bromine, 

 fluorine, calcium, potassium, strontium, magnesium, manga- 

 nese, &c, are ions, and that their electro-chemical equivalents 

 are the same as their ordinary chemical equivalents. 



840. Propositions iv. and v. extend our means of gaining 

 information. For if a body of known chemical composition 

 is found to be decomposable, and the nature of the substance 

 evolved as a primary or even a secondary result (743. 777.) 

 atone of the electrodes, be ascertained, the electro-chemical 

 equivalent of that body may be deduced from the known con- 

 stant composition of the substance evolved. Thus, when 

 fused protiodide of tin is decomposed by the voltaic current 



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