The reaction will be readily understood, if it be remembered what takes place
when water is decomposed by voltaic electricity. Hydrogen is obtained at the
negative and oxygen at the positive electrode^ because the latter consists likewise
of platinum, a metal which has no affinity for oxygen. Hence both constituents
of water, hydrogen and oxygen, are evolved at the respective poles. Now, if
instead of water, hydrochloric acid be decomposed in like manner, chlorine is
evolved at the positive electrode. If the positive electrode consists, however, of
a metal, such as Zn, Cu, Fe, for which chlorine possesses a strong affinity, the
latter combines with these metals, the moment it is liberated on the positive
electrode, or as it is termed in its nascent state {in statu nascendi) , and hydrogen
alone is evolved at the negative pole.
If in the place of hydrochloric or sulphuric a«id, a dilute solution of a
metallic chloride or sulphate, e.g., cupric sulphate, SOaCuo", is used, and the two
electrodes dipped into it, a red film or deposit of metallic copper is immediately
observed at the negative (platinum) electrode, whilst at the positive electrode,
consisting of metallic zinc, no evolution of gas is visible, since the zinc is acted
upon by the acid liberated from the copper, and is converted thereby into
BO^Zno . An equivalent quantity (65 parts by weight) of zinc remains dissolved
in the acid for the 63"5 parts by weight of copper, precipitated on the platinum
foil or crucible lid. In this manner the amount of copper present in a solution

F




Qiy CUPROUS OXIDE.
