He then suggests as a measurer of current the standard form of electrolytic cell ever since known as the _voltameter_. He preferred that kind in which water is decomposed, the quantity of electricity which had flowed through it being measured by the quantity of the gas or gases evolved during the operation. Before adopting this he undertook careful experiments in which his fine manipulative skill, no less than his chemical experience, was called into service to verify the fact that the quantity of water decomposed was really proportionate to the quantity of electricity which has been passed through the instrument. Having this standard, he investigated numerous other cases of decomposition by the current, and so arrived at a substantial basis for the doctrine of _definite electro-chemical_ action. Speaking of the substances into which electrolytes are divided by the current, and which he had called ions, he says: “They are combining bodies; are directly associated with the fundamental parts of the doctrine of chemical affinity; and have each a definite proportion, in which they are always evolved during electrolytic action.... I have proposed to call the numbers representing the proportions in which they are evolved _electro-chemical equivalents_. Thus hydrogen, oxygen, chlorine, iodine, lead, tin are _ions_; the three former are _anions_, the two metals _cations_, and 1, 8, 36, 125, 104, 58, are their _electro-chemical equivalents_ nearly.” This fundamental law being set upon an impregnable basis of facts, he goes on to speculate upon the _absolute quantity_ of electricity or electric power belonging to different bodies; a notion which only within the last few years has found general acceptance. In developing this theory he uses the following language:-- According to it [_i.e._ this theory], the equivalent weights of bodies are simply those quantities of them which contain equal quantities of electricity, or have naturally equal electric powers; it being the ELECTRICITY which _determines_ the equivalent number, _because_ it determines the combining force. Or, if we adopt the atomic theory or phraseology, then the atoms of bodies which are equivalents to each other in their ordinary chemical action, have equal quantities of electricity naturally associated with them. But I must confess I am jealous of the term _atom_....