glass vessel for bearing a brisk fire and sudden alterations of heat and
cold without breaking.
As the method of cutting these glass vessels is no where described in
books, I shall here give a description of it, that they may be made by
chemists for themselves out of spoiled retorts, matrasses, and
recipients, at a much cheaper rate than any which can be procured from
glass manufacturers. The instrument, Pl. III. Fig. 5. consisting of an
iron ring AC, fixed to the rod AB, having a wooden handle D, is employed
as follows: Make the ring red hot in the fire, and put it upon the
matrass G, Fig. 6. which is to be cut; when the glass is sufficiently
heated, throw on a little cold water, and it will generally break
exactly at the circular line heated by the ring.
Small flasks or phials of thin glass are exceeding good vessels for
evaporating small quantities of fluid; they are very cheap, and stand
the fire remarkably. One or more of these may be placed upon a second
grate above the furnace, Pl. III. Fig. 2. where they will only
experience a gentle heat. By this means a great number of experiments
may be carried on at one time. A glass retort, placed in a sand bath,
and covered with a dome of baked earth, Pl. III. Fig. 1. answers pretty
well for evaporations; but in this way it is always considerably slower,
and is even liable to accidents; as the sand heats unequally, and the
glass cannot dilate in the same unequal manner, the retort is very
liable to break. Sometimes the sand serves exactly the office of the
iron ring formerly mentioned; for, if a single drop of vapour, condensed
into liquid, happens to fall upon the heated part of the vessel, it
breaks circularly at that place. When a very intense fire is necessary,
earthen crucibles may be used; but we generally use the word
_evaporation_ to express what is produced by the temperature of boiling
water, or not much higher.
SECT. IV.
_Of Cristallization._
In this process the integrant parts of a solid body, separated from each
other by the intervention of a fluid, are made to exert the mutual
attraction of aggregation, so as to coalesce and reproduce a solid mass.
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