. This treatment of the numbers gives a mean result
for the six analyses of 7.68 per cent. of water, the amounts not
varying by more than 1 per cent.

It will be seen that the dehydration has never passed the composition
corresponding to 2 CaSO4 + H2O; indeed, the material approximates
more nearly to the composition 3 CaSO4 + H2O. It appears probable,
therefore, that in the successful preparation of plaster the whole, or
nearly the whole, of the gypsum is changed, but that this change does
not result in the production of CaSO4, or of a mixture of CaSO4 and
CaSO4 + 2 H2O, but of a lower hydrate of calcium sulphate.

In the case of the analyses, given by Landrin, of fine plaster for
potteries, the percentages of water (8.14 and 8.08) correspond closely
to that of a hydrate, 3 CaSO4 + 2 H2O, which would contain 8.1 per
cent. of water.

Some surprise may have been excited by the fact that the well known
method of revivifying hydrated calcium sulphate has recently formed
the subject of a patent (Eng. pat., No. 15,406).

The method described in the specification consists in reducing the
materials (waste moulds, etc.) to small lumps, and baking between the
temperatures of 95 deg. and 300 deg.. It is mentioned that the whole of the
water must not be expelled. This is no doubt correct, but it must be
effected by regulating the _time_ of baking, since by prolonging the
operation all the water of crystallization can be expelled far below
300 deg.. To secure even baking the mass is kept stirred by mechanical
stirrers, a necessary precaution, since the operation is to be carried
out in an ordinary kiln. The process is stopped when a portion of the
plaster is found to set in the required time, a method of regulation
which will probably be found to work well in practice.--_Chem. Trade
Jour._

* * * * *

SPACING THE FRETS ON A BANJO NECK.

BY PROF. C.W. MACCORD.

The amateur performer on the banjo, if he be of a mechanical turn, is
often tempted to exercise his skill by making an instrument for
himself; and the temptation is the greater because he can confine
himself to the essentials. The excellence of a banjo in respect to
power and tone depends mainly upon the rim and the neck, that is,
supposing the parchment head to be of proper quality; but then the
preparation of the heads is a business of itself, and the amateur is
no more expected to make the head than to make the strings. So ag