171 



CHAPTEK VII 

 VISCOSITY OF GASES 



73. On the Character of Internal and External 

 Friction 



BY the name of internal friction } Newton has denoted a 

 property of fluid media which is also termed viscosity. This 

 property exhibits itself in phenomena whose cause we might 

 be inclined to look for in a cohesion which is exerted during 

 motion and acts in opposition to motion, and perhaps we 

 should not be wrong in the case of liquids ; in gases, how- 

 ever, which have no sensible cohesion, we must look for 

 another cause, and perhaps therefore in liquids too. 



To push a solid body on a liquid at rest, or one liquid 

 layer over another, a certain force is required, just as force 

 must necessarily be employed when a solid body is rolled or 

 slidden along a solid support at rest. Analogy justifies the 

 use of the word friction to all these phenomena, and there- 

 fore the ascription of friction to fluid media and not alone to 

 solids. So by the external friction of a fluid we mean the 

 friction that is brought into play at the surface of separa- 

 tion of the fluid and a solid body or of two fluids, while as 

 internal friction we denote that friction which acts between 

 layers of one and the same fluid which move with different 

 speeds. 



It is not difficult to see the reason why force must be 

 employed to overcome this, which is perhaps only apparently 

 friction in the fluid. When a body is moved either in or 

 upon a fluid it puts the fluid also into motion, and thus loses 

 a part of its energy, just as by friction against a solid support ; 



1 ' Attritus vel resistentia quas oritur ex defectu lubricitatis,' Philosophies 

 Naturalis Principia Mathematica, 1687, lib. ii. sect. 9. 



