Chemical excitement is, in the greatest number of cases, the most advantageous; but it is necessary here to distinguish that which belongs to the horny hardening, from that which is the effect of irritability brought into action. 1st. Plunge a frog without skin and alive into a concentrated acid; instantly every thing is disorganized; the reagent acts so strongly, that we can distinguish neither horny hardening nor contractility. 2d. Weaken the acid a little and plunge into it, the inferior extremities only of a frog; in an instant they stiffen by the contraction of the extensors, which overcome the flexors; for in this experiment, this is almost a constant phenomenon; withdraw the animal; its thighs remain immoveable, life has been extinguished in them; the contraction that has come on is a horny hardening, and not a vital phenomenon. A dead frog plunged into the same liquor experiences the same phenomenon. 3d. Weaken the acid still more; the instant the animal is plunged into it its limbs contract; but relaxation succeeds the contractions; these are alternate motions; it is the irritability that begins to be put into action. Yet if the acid is not very weak, some marks of the horny hardening still remain, and the animal has a stiffness in the motions of the inferior extremities, the evident result of the first degree of this horny hardening. 4th. Finally, if the acid is very weak, it becomes a simple irritant which puts in action the sensible organic contractility, without altering the texture of the fibres; the animal after coming out of the fluid preserves the same power of motion. These experiments which it would be easy to multiply upon animals with warm blood, but which I have never attempted upon them, evidently show what belongs to the horny hardening, and what is the effect of vital contraction. Yet there is not an exact limit between them, and there is one degree of weakness of the acid in which these two causes of motions are confounded.