PHYSICAL PROPERTIES OF WATER 13 



Experiments by Hauser indicate that above 32 F. the viscosity of 

 water, in addition to varying with temperature, also increases very 

 slightly with an increase in pressure, and this would appear to be borne 

 out by the critical velocity experiments of Messrs. Barnes & Coker at 

 the Me Gill University. It may be noted that whereas with liquids the 

 viscosity decreases with an increase in temperature, with gases the reverse 

 is the case. Researches by Dr. Grindley and the author show that in the 

 case of air the viscosity is independent of the pressure, and, if temperatures 

 are measured in degrees Centigrade, increases with the temperature 

 according to the law 



p = {355-3 + 1-168 T - '00248 T 2 } X 10~ 9 ft. Ib. sec. units, or 

 approximately 



p= {358 + -932 T} X 10~ 9 ft. Ib. sec. units 

 between the limits of temperature C. and 100 C. 1 



This is as might be inferred from the kinetic theory of fluids, for layers 

 of fluid moving with different velocities are continually interchanging 

 molecules by diffusion. Thus the more rapidly moving layers are con- 

 tinually losing momentum by interchange with the slower layers, and a 

 continually applied force is necessary to maintain this state of motion. 

 Since the diffusivity increases with an increase of temperature both in 

 liquids and gases, it would be expected that in both cases the viscosity 

 would increase with temperature. 



It is, however, extremely probable that cohesion plays a great part in 

 producing viscosity in liquids, and while in gases the molecules exert 

 forces on each other by collision only, so that here diffusivity is the 

 important factor in producing viscosity, in liquids the greater cohesion 

 at lower temperatures more than counterbalances the diminished diffusivity 

 and hence increases the viscosity. 



Since the interchange of molecules will be proportional to the area 

 over which such interchange takes place, the resistance to distortion will 

 be proportional to this area. Evidently then if eddies are formed in the 

 course of a stream of fluid, since the area over which interchange of 

 momentum may take place is greatly increased, the viscous resistance to 

 motion will also be increased. 



Viscosity, being a physical property of a fluid, is independent of the 

 velocity of translation of its particles, and where motion takes place in 

 parallel straight lines, the resistance to motion can be directly inferred 

 from a knowledge of the viscosity. If eddies are formed, however, the 



1 " Proc. Roy. Soc.," vol. 80 A., 1908, p. 114. 



