Magnetic Rotation. 73 
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carbonate of copper, the white chiefly of oxide of tin, and the 
red of protoxide of copper in octahedral crystals, mingled with 
octahedrons of pure metallic copper. Beneath these substances 
the metal was quite bright, and it was found by analysis to 
consist of copper and 18°5 per cent of tin. A nail of a simi- 
lar alloy from a tomb at Ithaca, and a mirror from a tomb at 
Samos, in Cephalonia, presented the same appearances, but in 
less distinct crystallization: the mirror was composed of cop- 
per alloyed with about six per cent of tin, and minute portions 
of arsenic and zinc. A variety of ancient coins, from the ca- 
binet of a celebrated collector at Santa Maura, presented si- 
milar appearances, and afforded corresponding results; the 
white incrustations being oxide of tin, the green consisting of 
carbonate and submuriate of copper, and the red of the prot- 
oxide of the same metal; some having a dingy appearance ari- 
sing from the presence of black oxide of copper mingled with 
portions of the protoxide. Dr. Davy was unable to detect 
any relation between the composition of the respective coins 
and their state of preservation, the variation in this respect 
which they presented appearing to arise rather from the cir- 
cumstances under which they had been exposed to the mine- 
ralizing agents. In conclusion, Dr. Davy observed, that as 
the substance from which these crystalline compounds had been 
produced could not be imagined to have been im solution, their 
formation must be referred to an intimate motion of its particles, 
effected by the conjoint agency of chemical affinities, electro- 
chemical attraction, and the attraction of aggregation. He sug- 
gested the application of this inference to explain various phz- 
nomena in mineralogy and geology.— Annals of Philosophy. 
MAGNETIC ROTATION. 
M. Arago’s beautiful experiment is now well known, and, 
as it deserves, attracts attention every where. The following 
are some results obtained by MM. Prevost and Colladon, which, 
as they vary slightly in certain points from those as yet pub- 
lished in this country, will be interesting to such as pursue 
this branch of science. 
A disc formed of a thick copper wire rolled in a spiral, pro- 
duced much less effect than a perfect disc of the metal of the 
same weight and size. 
A disc of glass covered with lead, or a single leaf of tin glued 
on to wood, sensibly deviated the needle. Wood alone, or 
es or a disc of peroxide of iron, had no appretiable ef- 
ect. 
A disc of hammered copper deviated the needJe more 
strongly than the same disc annealed. 
Vol, 67. No. 333. Jan. 1826. K A screen 
