But, some one will doubtless say, how do you explain the stoppage in the motions of the camphor on the surface of water contained in vessels that are not perfectly clean? Before answering this question, let us say in the first place that the cause of the motions under consideration is due to nothing else but the evaporation of this concrete oil--to effluvia that escape from all parts and that exert upon the body whence they emanate a recoiling action exactly like that which manifests itself in an ælopile mounted upon a brasier, or, better yet, in the explosion of a sky-rocket. A portion of these camphory vapors, as well as a small portion of the camphor itself, dissolves in the water and forms upon its surface an oily layer which is at first very slight, but the thickness of which may increase in time until it becomes (especially if the vessel is narrow) a mechanical obstacle to the gyration of the small fragments of camphor that it imprisons, and whose evaporation it prevents. Now, as this layer of volatile oil may and does evaporate, in fact, after a certain length of time, the camphor then resumes its gyratory motions; but there is not the least reason in the world for saying on that account that it "has _habituated_ itself to the cause which had at first influenced it, and that, too, in modifying itself in such a way as to render null the influence of a cause that has not ceased to be present" (Dutrochet, _l.c._., p. 50). We have been enabled to convince ourself of the existence of this oily layer of camphor when it was of a certain thickness by introducing under the water on which it, had formed, a few drops of sulphuric ether whose sudden evaporation produced sufficient cold to instantaneously congeal the layer in question and thus render it perfectly visible to the eye. The slight layer of greasy matter that habitually lines the sides of vessels from whence no effort has been made to remove it, produces effects exactly like those of the oil of camphor, that is to say, that in measure as it becomes thicker it likewise arrests the motions of the concrete volatile essence.