900 REPORT—1885, 
afternoon. During the prevalence of land winds there are frequent local showers, 
but these are evidently not the cause of the negative readings. Negative readings 
are never obtained in fine weather, except when the wind is westerly, and not 
even then, unless the ground is dry. A shower of rain, which cools the ground, 
makes the succeeding readings positive for some hours, A marked feature during 
the prevalence of land winds are the great clouds of dust which fill the air, and it 
seems probable that there is some connection between these dust clouds and the 
negative electrification of the air. 
7. Molecular Distances in Galvanic Polarisation.} 
By Professor J. Larmor, M.A. 
It has been shown, principally by Helmholtz (‘ Wissen. Abhand.’ Vol. I.. 
Galvanismus, and Faraday Lecture, ‘ Journal of the Chemical Society,’ 1882), that 
polarisation involves a condensing action on the surface of the metallic electrodes. 
The particles, say of the cation, which exist in the electrolyte in a state of tem- 
porary dissociation, are drawn towards the cathode by the electromotive force till 
further approach is prevented by chemical forces. Thus along the surface of the 
cathode we obtain a sheet of positively charged cation molecules, which are in an 
equidistant arrangement on account of their mutual repulsion; and opposite to- 
each on the material of the electrode there is the equal and opposite charge drawn: 
there by electrostatic induction, This double sheet is equivalent to a condenser. 
Kohlrausch and Helmholtz measured the charge required to produce a given 
polarisation difference of potential, and therefrom estimated the thickness of this: 
double layer. 
Lippmann (‘Comptes Rendus,’ 1882) determined the surface energy of the 
charge for unit area from the variation of the capillary constant of a mercury 
electrode; and thereby gained an estimate in agreement with the above. 
We should expect that the capacity of the double layer per unit surface would 
remain constant until the distance between contiguous cation particles became of 
the same order as the thickness of the double layer, ¢.e., until the contiguous parti-- 
cles began to feel one another’s chemical forces, and be influenced thereby. An 
examination of Lippmann’s results shows this constancy of capacity to a very close: 
degree for a range of about a volt. We may, accordingly, conclude that with his. 
acidulated water polarised to a yolt the particles of the polarisation layer have 
just become so numerous as to be in chemical contact with one another. This 
eae to a third estimate of a molecular distance which we find to agree with the: 
two former. 
The first estimate is based on the absolute electrostatic measurement of the 
charge ; the second on the measurement of a surface tension; the third on the: 
absolute electro-chemical equivalent of the electrolyte. The complete agreement 
of three estimates founded on physical constants so various and of so different 
orders of magnitude, is strong evidence of the validity of this method of interpre- 
ting the phenomena of polarisation. 
They are all in satisfactory agreement with the estimates of Sir W. Thomson 
and others from different considerations (‘ Nature’ 1870, ‘ Appendix F, Thomson 
and Tait’s Natural Philosophy,’ part ii.), and give an average result of about one 
10°7° metre, more or less. 
8. On the Employment of Mance’s Method for eliminating the Effects of 
Polarisation, to determine the Resistance of the Human Body. By 
Dr. W. H. Stonz, M.A. 
The author commenced by stating that he had given a similar paper at the 
Southport meeting of the Association, in which it was noted that the electrical 
resistance of the human body was surprisingly less than that usually given, even 
by such authorities as Rosenthal, in Germany, and Dolbear, in America. He had 
1 See Phil, Mag. Nov. 1885, 
