214 Proceedings of the Cambridge Philosophical Society. 
sively either to the class of attractive or of repulsive double- 
refracting crystals, nor to the intermediate class, which is devoid 
of the property of double refraction. They appeared to belong 
at once to all the three classes of media just mentioned ; pos- 
sessing the property of attractive crystals, when exposed to the 
rays, forming one extreme of the spectrum, and of repulsive in 
their action on the other extreme ; while for certain interme- 
diate rays they were altogether devoid of the property of double 
refraction. Mr Hersche! was led to this inference, by observe 
ing that the curves, whose ordinates represented the polarizing 
energy, after approaching very rapidly to the axis, would again 
recede rapidly from it on the same side, except the ordinates 
were supposed to become negative, which appeared more pro- 
bable. This induced him to examine the truth of his supposi- 
tion, by measures taken in homogeneous light, and the result 
was a complete confirmation of the remarkable singularity above 
noticed. 
May 9A.—Qn the application of Magnetism as a Measure 
of Electricity. Bv the Rev. J. Gumming, F. R. S. F. C. P. S.j 
Professor of Chemistry. — After mentioning the difficulties in 
applying the decomposition of water or the fusion of wires, 
either as tests or as measures of galvanism;, the instrument de- 
scribed in a former paper by Professor Gumming, was shewn 
to be capable of detecting the galvanism developed bv two sur- 
faces not greater than the ^ J 0 th of an inch, and was applied to 
discover the galvanic action of different metallic surfaces and 
fluids; amongst others of zinc and potassium, and of strong 
and dilute sulphuric-acid. A galvanometer was described, con- 
sisting of a connecting-wire moveable upon a graduated slide. 
By comparing the deviations of a needle placed below it at dif- 
ferent distances, the tangent of the deviation was found to vary 
inversely as the distance of the connecting-wire from the mag- 
netic needle. On applying the deviation produced on the mag- 
netic needle as a measure of the increased effect produced by 
moving two galvanic surfaces towards each other, it appeared 
that the tangent of the deviation Varied inversely as the square 
root of the distance of the plates from each other. The latter 
part of the paper consisted of a detail of miscellaneous experi- 
ments. A steel-wire was made permanently magnetic, by twist- 
