CHAP, vi.] SOME OTHER SENSATIONS. 1417 



conscious of a specific sensation of cold when the temperature of a 

 region of the skin which has previously been fairly constant is 

 with sufficient rapidity lowered. To how large an extent we are, 

 under ordinary circumstances, unconscious of the actual tempera- 

 ture of the skin and how sensitive we are to even slight changes 

 of temperature may be illustrated by using one region of the skin 

 as a stimulus of heat or cold for another. At a time, for instance, 

 when we are not directly conscious of the hand being either 

 colder or hotter than the forehead, by putting the one up to the 

 other we may experience a distinct sensation telling us that the 

 hand decidedly differs in temperature from the forehead ; we feel 

 at once that one is warmer or colder than the other, though it 

 may take some little time to recognise which is the warmer or the 

 colder. 



880. These sensations of heat and cold behave very much 

 in the same way as sensations of pressure. We have already said 

 that the change of temperature like the change of pressure must 

 be effected with a certain rapidity in order to produce a distinct 

 sensation, and in general the more gradual the change the less 

 intense is the sensation. 



As might be expected from the fact that it takes a longer time 

 to produce a change of temperature than to exert pressure, the 

 sensation of either heat or cold is somewhat slowly developed and 

 lasts some considerable time ; hence consecutive sensations readily 

 fuse into one. 



Since it is the changed temperature and not the particular 

 temperature arrived at which is the basis of the sensation, a hot 

 body or a cold body gives rise to a sensation only at the first 

 contact or approach and for some little time afterwards, the effect 

 diminishing from the very moment that the change has been 

 established. Hence a hot body or a cold body, even when kept 

 itself at a constant temperature and not cooled or heated by 

 contact with the cooler or warmer skin, ceases after a while to be 

 felt as hot or cold. For this reason the repeated dipping of the 

 hand into hot or cold water produces a greater sensation than when 

 the hand is allowed to remain all the time in the water, though in 

 the latter case the temperature of the skin is most affected. 



The effects of contrast are obvious in sensations of heat and 

 cold as in those of pressure ; when the hand is dipped in hot water 

 the sensation is most intense at the ring where the hand emerges 

 from the surface of the water. 



We can with some accuracy distinguish small differences of 

 temperature, especially those lying near the normal temperature 

 of the skin. In this respect these sensations follow Weber's law, 

 though apparently slight differences of cold are more easily recog- 

 nized than those of slight heat. The range of the greatest sensi- 

 tiveness seems to lie between 27 and 33. 



The regions of the skin most sensitive to variations in tempera- 



