By greatest porosity best provision is made for the evaporation of perspiration, the quantity of which varies greatly under different conditions. In a day of rest the amount as determined by Seguin and Voit is 900 grams (about 1 quart). During exercise it may increase to quantities incredible, were the figures not furnished by the best of observers. For example: Dalton mentions its increase to 380 grams per hour! and Dr. Southwood Smith has seen it rise to 1,600 grams per hour during violent exercise in a heated atmosphere! Now, if a clothing possesses no porosity, _e. g._, the mackintosh, and rubber clothing generally, even without exercise, there would collect somewhere beneath it a quart of water, but if exercise be indulged in, the quantity may become large indeed; particularly after the atmosphere beneath has been surcharged with vapor, and evaporation ceases to occur from the surface, and with it the grateful cooling process. The French Government has not permitted its introduction into its army for such obvious reasons. Of course, for a short time during a shower they may and do prove useful; but I am convinced that many have incurred most serious injury, even death, by throwing off the rubber clothing after the inner clothing had become permeated with moisture, when the chilling, incident to the sudden increased evaporation, has resulted in some acute inflammation. Moreover, the evaporation of the normal cutaneous moisture (with that of the lungs) requires 750 heat units or one-fifth of all the heat produced in the system. (Dalton.) Conservation of part of this loss contributes an equivalent amount of force to the organism, since heat and force are interchangeable terms. _This can be done._ Under normal conditions evaporation of perspiration occurs in the "insensible," _i. e._, vapor state, but change of these conditions (increased heat, and moisture in the atmosphere, increased exercise, etc.,) causes it to collect upon the integument in the visible or sensible state, and unless conducted away, may chill the body. Prevention of such condensation will avoid such dangerous and deleterious influences. The cause of condensation is a lowering of the temperature. _We have simply to maintain its temperature until at a perceptible distance from the body. This can be accomplished by a layer of loose wool, such as is hereafter described. The "residual air" having been once raised to the body temperature, it remains so, and the vapor does not assume the liquid state until meeting with the chilling influences in the outer layer of cloth._ Finally, bodies passing from the gaseous to the liquid state emit the heat--latent heat--which was essential to their assuming the gaseous condition. This occurring in the case of perspiration in the cloth interstices increases by just so much their warmth, in other words lessens the demand for heat production.