188 Proceedings of the Cambridge PhMosopJiical Society. 
ped by crystals with one axis of double refraction on exposure 
tx> polarised light.” 
3. Observations on the Notation employed in the Calculus of 
Functions. By Charles Babbage, Esq. was read. His object was 
to give some account of the important consequences which had 
followed on the adoption of improvements in notation. He then 
explained, by elaborate examples, the advantages to be derived 
from the notation of functions in its present improved state. 
Last Meeting in the Easter Term, — May 15. — 1. A paper 
by Mr Emmet on the Mathematical Principles of Chemical 
Philosophy was read. 
% Dr E. D. Clarke read a paper On the Purple Precipitate of 
Cassius. He concludes from his experiments, 1. That the two me- 
tals which are chemically combined in the purple powder of cas- 
sius, do not exist in a constant relative proportion. % That in what- 
ever proportion the gold may exist with regard to the tin in the 
purple precipitate, there is no evidence to warrant the opinion 
of its being in the metallic state. 
3. Professor Sedgwick concluded his paper on the Geology 
of Cornwall, &c. by describing the schistose rocks and the sub- 
ordinate beds. 
4. A paper by Mr Christie was read On the Laws according to 
w^hich Masses of Iron influence Magnetic Needles.” Instead of 
the hypothesis, that a mass of iron, disturbing a needle by be- 
coming a magnet, having its north and south poles in the upper 
and lower part respectively, Mr Christie supposes the needle to 
be guided in its horizontal direction by magnetic particles pass- 
ing through its centre in the direction of its natural dip ; and 
the iron to act principally, if not wholly, on these particles, 
causing, by their deviation towards it, a corresponding devia- 
tion of the horizontal needle. In confirmation of this, he found 
by experiment, that when the disturbing mass is placed at the 
same distance from the magnetic axis and the centre of the 
needle, the deviation of the horizontal needle, when properly es- 
timated from the magnetic axis, is always the same, whether the 
mass be placed to the north, or the south, or any other point of 
the compass with respect to the needle. In a seiies of elaborate 
experiments, he shews the mean coincidence of their results with 
those to which the theory above described would lead. 
