570. x. Electro-chemical equivalents are always consistent; i.e. the same number which represents the equivalent of a substance 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.
571. xi. Electro-chemical equivalents coincide, and are the same, with ordinary chemical equivalents.
572. By means of experiment and the preceding propositions, a knowledge of ions and their electro-chemical equivalents may be obtained in various ways.
573. 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.
574. 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 (131), 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 (529), 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 volumes, 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 described in these Researches, that chlorine, iodine, bromine, fluorine, calcium, potassium, strontium, magnesium, manganese, etc., are ions, and that their electro-chemical equivalents are the same as their ordinary chemical equivalents.
575. 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 (478, 512) at one of the electrodes, be ascertained, the electro-chemical equivalent of that body may be deduced from the known con-
