This section is from the book "A Treatise On Therapeutics, And Pharmacology Or Materia Medica Vol1", by George B. Wood. Also available from Amazon: Part 1 and Part 2.
It is only when of higher degree than the temperature of the body, or of that part of the body to which it may be applied, that heat is used therapeutically. No precise degree, therefore, can be fixed, at which it will take rank among remedies; for the temperature of the body which must determine this point, is itself variable. But, as a general rule, it may be said that a heat of 95° F. or upwards, may be remedial; for this is above the average temperature of the surface of the body; and a decided sensation of heat will be excited by the contact of a conducting substance, at that temperature, with the skin. But, should the surface be very cold, a much lower temperature than that mentioned might prove stimulant; and, on the contrary, should it be very hot, a higher might be applied without any effect of the kind. It will be remembered, therefore, that when precise degrees may be mentioned hereafter, they are to be considered not as fixed, and applicable under all circumstances; but merely as an average, and of course to be varied with the particular condition of the body at any given time. It will be found also, in the progress of these observations, that the mode in which heat is applied influences very much the degree at which it will act as a remedy.