AT one time it was the fashion to ask “What is electricity?” and to taunt, or condole with, the man of science, because, said the questioner, he had no answer to offer. The question I always considered unreasonable, since we must take some fundamental entity, which we cannot explain in terms of other substances, as the basis of our theories, and electricity is an aspect of our fundamentals. It is not a kind of liquid, or a kind of gas, but something quite different from both. There should never be any question of calling electricity a fluid, and then expecting to deduce its complete behaviour from the known property of fluids. What we have rightly done is, rather, to find out by experiment what its actual properties are, and what general laws will cover them. It then proves that for certain limited purposes electricity can be considered as a fluid, but the limitations should always be borne in mind.