Sparking Distances between Plates for small Distances. 147 



Although in the above examples of non-electrolytic mixtures 

 and solutions there seems to be some connexion between the 

 values of m in the formula for the temperature-variation of 

 the viscosities of two liquids, and its value in the formula for 

 the viscosities of mixtures of the two, the number of instances 

 is too small to warrant a general conclusion being drawn. In 

 the case of the electrolytic solution (NaCl) the values o£ m for 

 the mixtures at different temperatures vary greatly from one 

 miother, although the values in the temperature formula are 

 practically identical. Possibly the magnitude o£ the heat of 

 solution of NaCl places it outside the class of simple mixtures 

 considered in what precedes. 



The result of this investigation may be summed up as 

 follows : — 



(1) No one of the three theoretical formulae (1), (2), (3) 

 represents the viscosity of a mixture with sufficient closeness. 



(2) The empirical formula I- J =v l l—\-\-v 2 [ — \ with a 



suitable value for m gives a satisfactory representation. 



(3) It leads also to Slotte's formula for the variation of 

 viscosity of a liquid with temperature. 



XT. The Sparking Distances between Plates for small Distances. 

 By Robert F. Earhaet *. 



HISTORICAL.— Previous to 1860, several series of 

 measurements on the difference of potential required 

 to cause a spark to pass between plates separated by a stratum 

 of air, had been made. These earlier experiments by Volta 

 (Identita, p. 53), Riess (Pogg. Ann. vol. xl. p. 333), and 

 others possess little more than historic interest, the means at 

 their disposal for measuring differences of potential having 

 been unreliable. The first trustworthy series of measure- 

 ments were those performed in 1860, under tbe direction of 

 Sir "William Thomson (now Lord Kelvin) (Electrostatics 

 and Magnetism, p. 250). In the series of measurements 

 referred to, the plates w r ere separated by known distances, 

 the distances being determined by a micrometer- screw. The 

 potentials required to cause a breaking down of the inter- 

 vening dielectric were measured by the absolute electrometer. 

 "The results obtained show that the difference in potential 

 necessary to break down the medium is not directly pro- 

 portional to the thickness of the separating medium. This 

 has been amply verified by other observers. Various 



* Communicated by the Author. 

 L2 



