1886.] 



The Coefficient of Viscosity of Air. 



315 



shells, by surface density multiplied by distance between the com- 

 ponent layers. When the two plates have attained electrical equili- 

 brium with surrounding objects, the potentials just outside the layers 

 will be equal (e.g., by electrolytic connexion as in Sir W. Thomson's 

 experiment of a zinc and a copper plate connected by a drop of water), 

 but the potentials of the metals inside will be different, each being 

 less by the moment of its layer. The zinc will therefore be at a 

 lower potential than the copper. If a metallic connexion is now 

 made between the metals, positive electricity will flow from the 

 copper to the zinc until the potentials of the metals are equalised, 

 and the difference of potentials of the air outside the plates will now 

 be exactly opposite to that which subsisted between the metals before 

 metallic contact. 



There is this difference between the view just mentioned and that 

 of pure contact difference which was examined, § 37, that here the 

 metallic connexion involves a running down of electric energy, while 

 there it involved an addition of energy from without. 



41. The explanation by superficial double layers once accepted, 

 there is no longer any reason for retaining contact of the two metals 

 as one of the conditions necessary for the setting up of this action 

 between a metal and its surrounding atmosphere. 



We may hold that when a piece of clean zinc, or a piece of clean 

 copper, kept insulated, is introduced into an atmosphere capable under 

 favourable circumstances of acting chemically upon it, a change of a 

 chemical or semi-chemical character is thereby initiated over its 

 surface, which involves a transformation of energy, though in minute 

 quantity, and which proceeds until a state of equilibrium is estab- 

 lished, as indicated by the setting up of a certain definite difference 

 of electrical potential between the metal and the gas near it, but out- 

 side the sphere of the action. 



VI. " The Coefficient of Viscosity of Air. Appendix.." By 

 Herbert Tomlinson, B.A. Communicated by Professor 

 G. G. Stokes, P.R.S. Received November L5,"l886. 



(Abstract.) 



In the previous experiments by the author on this subject the 

 coefficient of viscosity of air was determined from observations of the 

 logarithmic decrement of amplitude of a torsionally vibrating wire, 

 the lower extremity of which was soldered to the centre of a hori- 

 zontal bar. From the bar were suspended vertically and at equal 

 distances from the wire a pair of cylinders, or a pair of spheres. The 

 distances of the cylinders or spheres from the wire were such that the 



