When the excitability is in such a state as to be very susceptible of the action of external powers, I shall call it _abundant_, or _accumulated_; but when it is found not very capable of receiving their action, I shall say, it is _deficient_, or _exhausted_. I would not wish however, to have it thought, that by these terms I mean in the least to hint at the _nature_ of excitability, nor that it is _really_ one while increased, and at another diminished in quantity, for the abstract question is in no shape considered; we know not whether the excitability, or the vital principle, depends on a particular arrangement of matter, or from whatever cause it may originate; by the terms here used, I mean only to say, that the excitability is easily acted on when I call it abundant, or accumulated; at other times the living body is with more difficulty excited, and then I say, the vital principle is deficient, or exhausted. The laws by which external powers act on living bodies, will, on a careful examination, be found to be the following-- First, when the powerful action of the exciting powers ceases for some time, the excitability accumulates, or becomes more capable of receiving their action, and is more powerfully affected by them. If we examine separately the different exciting powers, which act on the body, we shall find abundant confirmation of this law. Let us first consider Light; if a person be kept in darkness for some time, and be then brought into a room in which there is only an ordinary degree of light, it will be almost too oppressive for him, and appear excessively bright; and if he have been kept for a considerable time in a very dark place, the sensation will be very painful. In this case, while the retina, or optic nerve, was deprived of light, its excitability accumulated, or became more easily affected by light; for if a person goes out of one room, into another which has an equal degree of light, he will feel no effect. You may convince yourselves of this law by a very simple experiment--shut your eyes, and cover them for a minute or two with your hand, and endeavour not to think of the light, or of what you are doing; then open them, and the day-light will for a short time appear brighter. If you look attentively at a window, for about two minutes, and then cast your eyes upon a sheet of white paper, the shape of the window-frames will be perfectly visible upon the paper; those parts which express the wood-work, appearing brighter than the other parts. The parts of the optic nerve on which the image of the frame falls, are covered by the wood-work from the action of the light; the excitability of these portions of the nerve will therefore accumulate, and the parts of the paper which fall upon them, must of course appear brighter. If a person be brought out of a dark room where he has been confined, into a field covered with snow, when the sun shines, it has been known to affect him so much, as to deprive him of sight