462 On the Viscosity of Liquids. 



a constant rate of shearing in the liquid much more satisfac- 

 torily than if we observe an oscillating motion such as that 

 of Coulomb's disk or Helmholtz's sphere. The existence of 

 sliding-friction can be directly tested, not only between mer- 

 cury and the more highly viscous liquids, but also between 

 any two liquids that do not dissolve each other. Thus spheres 

 of water may be used with nearly all the fixed oils, and 

 spheres of oil of cloves (density 1/0475) or of oil of myrrh 

 (density 1*0189) may be used with water. 



The apparatus is simple and inexpensive ; results may be 

 rapidly obtained when a few standard mercury spheres are 

 preserved. They should be kept in a sample of the viscous 

 liquid to be tested. If a sphere breaks, the pieces should be 

 washed in water and reunited on hard pressed blotting-paper. 

 The quality of oils is often tested by their viscosity, and 

 special viscosimeters on the capillary-tube principle of 

 Poiseuille are used for the purpose. A time-reading through 

 a sample of the oil with a standard mercury sphere offers an 

 expeditious way of testing. If the oil is thin and the mercury 

 falls too fast, a calibrated water sphere may be used instead. 

 A sphere of water of 1 millim. radius, coloured with eosin to 

 be clearly visible, travels at the rate of one inch per hour in 

 castor-oil at 8° C. ; and here in parenthesis it may be added 

 that we have by far the simplest method of observing the 

 time-integral of temperature for small ranges. 



The general method cannot be employed for opaque liquids, 

 as we wish to observe the falling sphere ; but it is probable 

 that with a little ingenuity this difficulty could be overcome 

 if the opaque liquid presented itself for examination. The 

 small inertia of the falling sphere, advantageous as it is in 

 exhibiting the slightest variations of temperature, is a serious 

 objection if small solid particles are held in suspension in the 

 liquid. As a rule these particles will avoid the small sphere 

 and not touch it ; but in the event of contact occurring 

 there is the likelihood of a permanent union between the two 

 and of the particle being dragged down with the sphere, with 

 consequent loss of speed of the latter. Hence clear liquids 

 must be used. 



The author wishes to acknowledge his obligations to Prof. 

 Henrici for very kindly rendering available the apparatus of 

 his laboratory for the needs of the above experiments. 



—■ _»is*r 



