148 HEAT AND MOTION. 



tain range : though there are cases in which the 

 melting and the passing into vapour follow each 

 other so very rapidly that the process of melting is 

 not usually observed. 



It is equally probable that in every motion what- 

 ever, even in the most gentle and cool that it is pos- 

 sible for us to imagine, there are the elements both 

 of sensal heat and of visible light; but that these only 

 become apparent when there are certain degrees of 

 resistance to the motion. Two pieces of dry wood, 

 rubbed against each other, soon become heated, and 

 they are not very long in taking fire, and burning 

 with light. But they do not heat so soon if they are 

 wetted, or covered with oil, or with any thing else 

 that lessens the resistance they have to the motion. 

 We feel the same truth in our own bodies. When 

 all the systems of vessels in which the blood and 

 other fluids circulate or move are in a healthy state, 

 we feel no sense of heat from the various motions, 

 though all of them are continual, and many of them 

 are rapid ; but when any part is so diseased as that 

 the motion is resisted, we then feel heat as well as 

 pain : and if the disease is only a whitlow, or some- 

 thing of an equally local nature, we feel the part as 

 hot as if it were burning ; and the feeling is not a 

 ^fiereiy inward feeling, like that of pain, it is an actual 

 increase of temperature, which we can discover by 

 the healthy hand, or measure by the thermometer* 

 just in the same way as if it had been communicated 

 by holding the part near to a common fire. In cases 

 of fever, the sense of heat is general all over the 

 body, and it too is discoverable by the touch of an- 

 other person or by the thermometer. 



In all these cases it is resistance to motion that 

 causes the heat to appear ; and the heat is always 

 in proportion to the motion and the resistance 

 jointly. Local inflammation, such as that of whit- 

 lows, is most common in young persons, in whom 

 the circulation is quick ; and fever is more severe 



