The Romance of Modern Chemistry

CHEMISTRY AND ELECTRICITY which might be regarded as responsible for the electric current. Closer examination, however, shows that the current has been obtained only at the expense of certain alterations in the cell. If, before allowing the cell to run, say for an hour, we were to weigh the two poles, then, on weighing them again afterwards, we should find that the zinc pole had become lighter, and the copper pole heavier. Further, we should find that the solution round the zinc pole contained more sulphate of zinc than at the start, and that the solution in contact with the copper pole had lost some of its copper sulphate. The changes, then, which occur during the production of the current are (1) the disappearance of some of the zinc to form zinc sulphate, and (2) the deposition of copper on the other pole from the copper sulphate. All this might be represented very simply in the following way: zinc + copper sulphate-*copper + zinc sulphate, the arrow indicating that the sub- stances on the left are replaced by the substances named on the right. This may strike the reader as something quite novel, but as a matter of fact a chemical change of exactly the same kind has already been considered in earlier chapters. One thing which, as was pointed out, served to support the alchemists’ belief in the transmutation of metals was the observation than when a clean steel knife-blade has been dipped into a solution of copper sulphate it looks as if it had been converted into copper. Things, how- ever, are not always what they seem, and careful investiga- tion has shown (1) that the formation of copper is only superficial, and (2) that in exchange for the copper which has spontaneously settled on the blade, a certain quantity of iron has passed into solution as sulphate of iron. The 293