74 Necessity of Water in the Preparation of Lead-plaster. 
A screen of copper, or copper and zinc interposed, dimi- 
nished the effect without destroying it. The diminution was 
greater as the screen was thicker, or placed nearer to the 
needle. A screen of glass had no influence. If the interposed 
metallic screen were pierced by an aperture equal in diameter 
to the length of the needle, its effect was very nearly the same. 
A vertical magnet suspended in the centre of a cylinder of 
copper remained unmoved, whatever the direction or rapidity 
of rotation of the ring. 
When two needles were fixed together in a similar direc- 
tion, the effect increased; when they were placed with their 
opposite poles together, it ceased entirely. 
A needle magnetized, so as to have similar poles at its two 
extremities, was the apparatus most sensible to the motion of 
the discs. It was one of this kind which the authors used in 
their delicate experiments. 
The conclusion arrived at by MM. Prevost and Colladon 
is, that the effects are due to a transient magnetization of the 
discs, which, not being able to modify itself with a rapidity 
proportional to that by which the different points of the disc 
are displaced by rotation, are transported to a small angular 
distance from the needle before they are changed, and draw 
it after them. This is the same explanation in effect as that 
of MM. Herschel and Babbage. 
Experiments made with care to determine the influence of 
the velocity and the distance of the discs, indicated that the 
angles of deviation, and not their sines, augmented proportion- 
ally with the velocity, at least, within certain limits, and that the 
sines of the angles of deviation increased in an inverse ratio of 
the power 2,4; of the distance. They were careful to employ, 
in this determination, discs having diameters very great in 
comparison to the length of the needle.— Bzb. Univ. xxix. 316. 
NECESSITY OF WATER IN THE PREPARATION OF LEAD-PLASTER. 
Attempting to form lead-plaster, the Emplastrum Plumbi of 
the Pharmacopeia, without the use of water, steam being the 
source of heat, I was surprised to find after several hours, du- — 
ring which time the litharge and oil had been kept at a tem- 
perature of 220°, or thereabout, and constantly stirred, not 
the slightest appearance of combination ; upon the addition of 
a small quantity of boiling water, the oil and oxide imme- 
diately saponified : water appeared, therefore, to be essential to 
the formation of the plaster. It also appeared probable the 
oxide might be in thestate of hydrate. To ascertain if such were 
the case, I precipitated, by potash, the oxide from a quantity 
of acetate; the precipitate, when washed, was dried by a ro 
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