172 PHENOMENA DEPENDENT ON MOLECULAR PATHS 73 



and if this loss is to be replaced force must be employed. 

 Just in the same way, a fluid which streams along the 

 surface of a movable solid or of a second fluid loses part of its 

 speed by imparting motion to that other solid or fluid body. 



What was said about external friction in these cases 

 applies also to the internal friction of fluids. If inside a 

 fluid one layer moves more quickly than its neighbours, it 

 drags them along with it, and loses a part of its speed by 

 giving it up to them, just as a moving body loses speed by 

 friction with its support. 



The friction of fluids, therefore, both internal and ex- 

 ternal, consists only in a transfer of motion ; but this 

 transfer does not proceed without loss : a part of the 

 translatory motion of the layers is transformed into heat, 

 and, since this change into heat is continually going on, the 

 motion of translation is in time all changed into heat- 

 motions, and mechanical motion is annihilated as in the 

 case of friction between hard bodies. This change into heat 

 becomes complete at once if the body on whose surface the 

 friction is exerted is fixed and immovable. 



This transformation into heat, also, is easy to understand. 

 Heat-motion differs from translatory motion only in the 

 particles moving in all possible directions, without dis- 

 tinction, and not, as in the latter case, all in one and the 

 same direction. Change, therefore, of mechanical motion 

 into heat-motion consists in nothing else than a change 

 of the direction in which individual particles move. A 

 multiform change of direction of this kind cannot fail to 

 occur in the crowd of particles of which the medium con- 

 sists if these particles exert actions upon each other either by 

 forces of cohesion or by collisions ; and therefore, along with 

 the transfer of speed, which is called friction, there must 

 also occur a partial transformation into heat. 



74. Newton's Fundamental Law of Internal 

 Friction 



The force which causes the transfer of motion from one 

 layer to another when internal friction occurs in a fluid has 



