The external covering of the hair appears to be of the nature of the epidermis. It has in fact almost all the attributes of it. 1st. The hairs of the head burn exactly like this membrane, give out when burning an analogous odour, and leave after combustion a similar kind of coal; now it is principally to the external portion that these phenomena are owing. 2d. Water penetrates the hairs with great ease; hence very useful hygrometers can be constructed with them; now the same is true of the epidermis; and moistened hairs in foggy weather present in this respect a phenomenon analogous to that of the epidermis softened, wrinkled and whitened by the contact of a cataplasm. 3d. It is by means of the epidermoid covering that the hairs are foreign to life, that they are insensible and never become the seat of any acute or chronic affection. 4th. This covering is white, whatever may be the colour of the hairs. The cause of the colour resides in the internal medulla; thus the epidermis of negroes and that of white people differ but very little. Hence why when the internal substance of the hair has disappeared, the canal remaining alone exhibits a more or less evident whiteness. 5th. In this state, though the interior of the hair may be dead, the epidermoid exterior, which is independent of it, preserves most commonly the faculty of growing when it is cut; thus the cutaneous epidermis is truly foreign to all the subjacent diseases of the skin. 6th. I presume that it is this covering which gives to the hairs of the head the property of remaining so long uninjured. When removed far from the access of the air, they remain unaltered for ages; they have not in them the principle of decomposition of the other animal substances. They never become putrid either in air or water. Thus we have seen that the cutaneous epidermis never undergoes putrefaction, which seizes upon the subjacent parts. It appears however that the hairs are more unalterable than the epidermis, and that there is even a difference of nature between them. In fact, 1st, maceration and ebullition, which make the epidermis very easy to be broken, though they soften it but little, leave the hairs with their usual resistance, unless carried to degrees that I have not tried. By boiling and macerating them comparatively with the epidermis, we easily make this observation. 2d. The acids act less efficaciously upon the hairs than upon this membrane; but the alkalies dissolve them with as much and even more ease. 3d. A thread of epidermis of equal thickness would be incomparably less resisting than a hair. 4th. The hairs can, like the epidermis, be painted of different colours; but they do not retain them so long, and on this account the colour must be renewed oftener.