The skin absorbs substances in watery solutions with difficulty, on account of the oil lying upon and in the epidermis; substances dissolved in oil and rubbed in are more rapidly absorbed; absorption takes place rapidly after the skin has been washed with ether, chloroform, or alcohol. =The Physiologic Effects of Water.=—These depend on whether the water is taken internally or applied externally. If applied externally, the effects will depend upon the temperature, whether hot, tepid, or cold; also on the manner of application, but, most of all, on the length of time which it is applied and the state of health of the individual. The temperature of water is classified as very cold, from 32° to 55° F.; as cold, from 55° to 65° F.; cool, from 65° to 80° F.; tepid, 80° to 92° F.; warm, 92° to 98° F.; hot, 98° to 104° F.; very hot, 104° F. and above. _Heat._—A general hot bath produces dilatation of the vessels of the skin and contraction of the vessels of the brain; a general activity of the glands of the skin, both perspiratory and sebaceous. Perspiration may be produced, either to the degree of slight moisture or of profuse sweating, according to the length of time and the intensity of the application made. In a very hot bath the rate of perspiration may be increased from fifty to sixty times the ordinary amount. The most pronounced effect possible may be secured by either the electric light or sun bath. Loss of fluids from the body has a depressing effect similar to, though somewhat less marked than, that produced by bleeding, so that there is a vital necessity for administering water internally before, during, and after the bath. Prolonged and repeated perspirations induced by artificial means weaken the skin, and thus lessen its power to resist cold impressions, unless counteracted by frequent cold applications. The general and usual reactions following the applications of heat are atonic and depressing in character. For the most part, the reactions following cold applications are to be preferred to those following hot ones. The three great vascular areas of the body are the muscles, the portal system, and the skin. Each of these parts may be regarded as a great reservoir, capable of retaining a large part of the entire amount of blood of the body. When one of these areas is in a state of congestion, the others must be in a comparative state of anemia. The restorative effects due to the application of heat are due partly to the elimination of fatigue poisons which is thereby encouraged, as well as by the reflex stimulation of the nerve-centers. The good effects are much more decided and lasting, and the exhausting effects neutralized, if the hot application is followed by a short cold one. The effects of a neutral bath, 92° to 95° F., is sedative, diminishing nervous irritability.