The slow and insensible horny hardening, the effect of the contact of neutral salts, such as alum, the muriate of soda, &c. of the air and of alkohol, offers a very different phenomenon from the first. It is not altered by the continued action of the cause that produced it, however long it may be; it does not soften in a slow and insensible manner, as it has hardened, but remains always firm and contracted. Are these two species only different degrees of the same, or are they derived from separate principles? I know not. I have only observed, that when the living solids have undergone the slow and gradual horny hardening, they are still susceptible of the other. We know that after many years drying, animal textures are hardened, as in a recent state, by the action of fire; I have made the same observation with regard to boiling and the acids. Textures that have been contracted for a long time by alkohol and the neutral salts present the same phenomenon. All animal textures are susceptible of sudden hardening, except the hair, the epidermis, and the nails; these, if we may so say, exhibit only the rudiments of it. In general the hardening is more sensible, in proportion as the fibrous texture predominates in the organs. Hence the muscles, the tendons, and nerves, are the most susceptible of it. The organs not fibrous, as the glands, show the least degree of it. The slow and insensible horny hardening is almost the same in every part. Both exist in textures deprived of animal contractility, of sensible organic contractility, and of contractility of texture, as well as in those which enjoy them in the highest degree. Thus the tendons, the aponeuroses, the bones, even when their calcareous substance has been removed by acids, may be hardened as well as the muscles and the skin. This single circumstance would suffice to distinguish the contractility arising from hardening from all others, if a variety of differences, which I shall point out hereafter, under the article of the muscles in particular, did not. When a texture is suddenly hardened, it loses more than half its length, and becomes twisted in various ways. Taken suddenly from the acid or boiling water, it remains hardened; but if it is pulled, it becomes elongated, and then contracts again, when the force applied ceases, so that it has acquired a real elasticity by the process of hardening. This elasticity is remarkable in the tendons, nerves and muscles, which before this are absolutely destitute of it. This elasticity is not an effect of the slow and insensible hardening of alkohol and the neutral salts. By macerating for a length of time the organized textures, they gradually lose the power of contracting suddenly, which does not, however, entirely disappear, until the maceration has reduced the textures to mere pulp.