144 HEAT AND MOTION. 



possible 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 be- 

 come 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 something of 

 an equally local nature, we feel the part as hot as 

 if it were burning ; and the feeling is not a merely 

 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 communi- 

 cated 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 top is discoverable by the 

 touch of another 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 

 and burning in the robust than in the weak. 



