Two points remain to be mentioned. One has reference to the conductivity of pure water. It is no easy matter to prepare pure water; even after the water has been distilled, it contains traces of substances which are ionised, such as carbonic acid or ammonia. It is possible, by employing special precautions, to add to the water before distillation substances which form non-volatile compounds with these impurities, and by making use of vessels of gold or platinum, which are not attacked, however slightly, by water, to produce almost pure water. Such water is not wholly devoid of conductivity, but its resistance to the passage of electricity is very great. It must be presumed that the water is for the most part molecules of H₂O, or perhaps even more complex molecules, such as H₄O₂, H₆O₃, etc. But there are, besides, a few + ions of H and OH ; so that water is capable of reacting in certain cases where ions might be suspected.