ience 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.
When the particles of a body are only separated by caloric, and the
substance is thereby retained in the liquid state, all that is necessary
for making it cristallize, is to remove a part of the caloric which is
lodged between its particles, or, in other words, to cool it. If this
refrigeration be slow, and the body be at the same time left at rest,
its particles assume a regular arrangement, and cristallization,
properly so called, takes place; but, if the refrigeration is made
rapidly, or if the liquor be agitated at the moment of its passage to
the concrete state, the cristallization is irregular and confused.

The same phenomena occur with watery solutions, or rather in those made
partly in water, and partly by caloric. So long as there remains a
sufficiency of water and caloric to keep the particles of the body
asunder beyond the sphere of their mutual attraction, the salt remains
in the fluid state; but, whenever either caloric or water is not present
in sufficient quantity, and the attraction of the particles for each
other becomes superior to the power which keeps them asunder, the salt
recovers its concrete form, and the cristals produced are the more
regular in proportion as the evaporation has been slower and more
tranquilly performed.

All the phenomena we formerly mentioned as taking place during the
solution of s