THE ADEQUATE STIMULUS FOR HEAT AND COLD. 951 



only by the temperature of the medium, e.g. air, in contact with its 

 surface, but also by other conditions of the medium, in so far as they 

 affect its power of abstracting (or of supplementing) the skin's heat. 

 This power will in any substance depend not only on (1) its tempera- 

 ture, but also on (2) its thermal conductivity, (3) its specific heat, and, 

 in the case of any particular object, (4) on the nature of its contact 

 with the skin, and (5) its interference with, or addition to, radiation. 

 The degree to which an object acts as a "cold" stimulus increases 

 cceteris paribus with the degree to which it is cooler than the skin 

 it acts on. Thus water at 20 is a stronger " cold " stimulus than 

 water at 25, but it does not follow that it is a stronger " cold " 

 stimulus than mercury at 25, although certainly a stronger "cold" 

 stimulus than oil at 25. That is, the activity of the object as a 

 " cold " stimulus depends on its thermal conductivity as well as on 

 its temperature. Hence, of water and oil at the same tempera- 

 ture (below the skin temperature), the latter is cceteris paribus 

 the weaker stimulus, because it conducts less ; of water and 

 mercury at the same temperature (below the skin temperature), 

 the former is cceteris paribus the weaker stimulus, the latter 

 conducts better, and more than sufficiently to counteract its lower 

 specific heat. Capacity for heat is also a factor. 1 The nature of 

 the contact is also a determinant of the intensity of the stimulus ; 

 continuous surfaces of contact are more effectual than are broken. 2 

 Also, of course, renewal of contact is very important. The very air 

 around us in a room, the temperature of which, the skin having 

 become thermally adapted to it, is giving us no sensation, does, if it 

 move more quickly over a skin-surface, very readily produce sensation 

 of cold, i.e. we " feel a cold draught." Sensation of cold can also occur 

 without contact with cold substances, namely, by increased radiation. 

 Thus we may feel bitterly cold in a room the temperature of which 

 is high, if the walls are nevertheless very cold, e.g. in a moun- 

 tain hut. In harmony with these facts are others regarding the 

 " adequate temperature." Given an area of skin fully adapted " for the 

 conditions of heat-supply and heat-loss under which it is living, it then, 

 ex deflnitione, produces no sensation either of " cold " or " warmth." An 

 object which, when in contact with that skin, evokes no sensation either 

 of " cold " or " warmth," is of the " adequate temperature " (Hering). 

 But the " adequate temperature " is not one definite temperature ; it is a 

 short range of temperatures above and below a mean. This range is 

 far greater for oil than for mercury. The better heat-conductivity of 

 the latter makes it difficult to so adjust it to the temperature of an 

 area of skin, e.g. finger, that it shall not in unit time abstract from or 

 add to the heat of skin sufficiently to cause a perceptible lowering or 

 raising of the temperature of the end-organs. The experimental 

 difficulty is increased by variation of the adequate temperature of 

 different areas of skin, and also of one and the same area of skin. 3 

 These facts all point to the intensity of a "cold" stimulus of given 

 contact area being dependent on the amount of heat which it can with- 

 draw from the skin in unit time ; in other words, on the degree to which 



1 Bering, Hermann's "Handbuch," Leipzig, 1880, Bd. iii. Abth. 2. 



2 Hering gives numerous examples of this, ibid. 



3 Thus Hering found the adequate temperature of the little finger regularly somewhat 

 higher, and that of the index ringer regularly somewhat lower, than that of the middle 

 finger, ibid. 



