692 Prof. C. Kinsley on 



seemed so conclusive that it was not thought necessary to 

 try a third. 



A test of Poiseuille's law for liquids in a constant physical 

 state being the chief object of the present inquiry, the effect 

 of impurity at very low rates of shear may be left for closer 

 study at a future date. 



Conclusions. 



1. Poiseuille's law holds true to within 1 per cent, for 

 kerosene and pure water between rates of shear of 5 and 

 500,000 radians per second in the case of water, and between 

 0*2 and 20,000 radians per second in the case of kerosene ; 

 or the coefficients of viscosity of those liquids are constant 

 within these limits. 



2. Very minute quantities of the constituents of glass 

 dissolved out by water have very large effects on the 

 viscosity of water at low rates of shear. 



3. Differences of rates of shear (at least within the above 

 limits) do not seem to account for the lack of perfect 

 agreement between different methods of measuring viscosity 

 and between different methods of testing the effect of electro- 

 static stress on viscosity; water and kerosene (within the 

 above limits) do not afford any support for the view that at 

 low rates of shear liquids may approximate in condition to 

 plastic solids. 



LXIV. Short Spark-Discharges. By Cakl Kinsley, 

 Assistant Professor of Physics , University of Chicago*. 



HPHE relation between spark-potential, distance, and the 

 A pressure of the gas used in the discharge-chamber was 

 obtained by Paschen f from a long series of experiments. His 

 conclusion has been reaffirmed by Carr % from a most satis- 

 factory series of observations which covered a much wider 

 range. He found that in all the gases examined, at any par- 

 ticular potential-difference used, the product of the gas- 

 pressure and the length of the spark-discharge is a constant. 

 Thomson §, in developing a theory of the spark-discharge, 

 has found that the above relation can probably be extended 

 to include all gases and "expressed by an equation of the 



Communicated by Prof. J. J. Thomson. Bead before the American 

 Physical Society, Chicago Meeting, 1905. 



t Paschen, TVied. Ann. vol. xxxvii. p. 379 (1889). 



t Carr,Proc.Eoy. Soc. vol. lxxi. p. 374 (1903). 



§ J. J. Thomson* < Conduction of Electricity through Gases/ pp. 374 

 et seq. 



