PHARMACY. 
and every attempt hitherto made to im-' 
prove it, by increasing the number of its 
apertures, have in reality injured it. 
Heat may be applied to vessels employ- 
ed in chemical operations, directly, as in 
the open fire and reverberatory furnace: 
or through the medium of sand j the sand- 
bath : of water ; the water-bath : of steam ; 
the vapour-bath : of air ; as in the muffle. 
Changes produced by chemical processes. 
These consists chiefly in a new mode of 
aggregation, combination, and decomposi- 
tion. 
The form of aggregation may be altered 
by fusion, vaporization, condensation, con- 
gelation, and coagulation. 
Fusion is the conversion of a solid into a 
liquid by the sole agency of caloric. Sub- 
stances differ very much in the degrees of 
their fusibility ; some, as water and mer- 
cury, existing as fluids in the ordinary tem- 
peratures of the atmosphere ; while others, 
as the pure earths, cannot be melted by any 
heat we can produce. 
Liquefaction is commonly employed to 
express the melting of substances, as tallow, 
wax, resin, &c. which pass through inter- 
mediate states of softness before they be- 
come fluid. Fusion is the melting of sub- 
stances which pass immediately from the 
solid to the fluid state, as the salts and 
metals, except iron and platinum. 
When, in consequence of fusion, the sub- 
stances operated on acquire a greater or 
less degree of transparency, a dense uni- 
form texture, and great brittleness, and 
exhibit a conehoidal fracture, with a spe- 
cular surface, and the edges of the frag- 
ments very sharp, it is termed vitrification. 
In general, simple substances are less 
fusible than compounds ; for example, the 
simple earths cannot be melted singly, but 
when mixed are easily fused. The addi- 
tions which are sometimes made to refrac- 
tory substances, to promote their fusion, are 
termed fluxes : which fluxes are generally 
saline bodies. 
Thus, the alkalies potash and soda pro- 
mote powerfully the fusion of silicious 
stones ; but they are only used for accurate 
experimehts. The white flux is a mixture 
of a little potash with carbonate of potash, 
and is prepared by deflagrating together 
equal parts of nitrate of potash and super- 
tartrate of potash. When an oxide is at 
the same time tn be reduced, tlie black 
flux is preferred, which is produced by the 
deflagration of two parts of super-tartrate 
of potash, and one of nitrate of potash. 
It differs from the former only in containing 
a little charcoal. Soap promotes fusion by 
being converted by the fire into carbonate 
of soda and charcoal. 
Aluminous stones have their fusion greatly 
promoted by the addition of sub-borate of 
soda. 
Bluriate of soda, the mixed phosphate 
of soda and ammonia, and other salts, are 
also occasionally employed for the same 
purpose. 
An open fire is sufficient to melt some 
substances, others require the heat of a 
furnace. 
The vessels in which fusion is performed, 
must resist the heat necessary for the 
operation. In some instances an iron or 
copper ladle or pot may be used, but most 
commonly crucibles ,are employed. These 
are of various sizes. The large crucibles 
are generally conical, with a small spout for 
the convenience of pouring out ; the small 
ones are truncated triangular pyramids, 
and are commonly sold in nests. The 
Hessian crucibles are composed of clay and 
sand, and when good, will support an in- 
tense heat for many hours, without soften- 
ing or melting; but they are disposed to 
crack when suddenly heated or cooled. 
This inconvenience may be on many occa- 
sions avoided, by using a double crucible, 
and filling up the interstice with sand, or 
by covering the crucible with a lute of clay 
and sand, by which means the heat is trans- 
mitted more gradually and equally. Those 
which ring clearly when struck, and are of 
an uniform thickness, and have a reddish 
brown colour, without black spots, are 
reckoned the best. Wedgewood's cruci- 
bles are made of clay mixed with baked 
clay finely pounded, and are in every re- 
spect superior to the Hessian, but they are 
very expensive. The black lead crucibles, 
formed of clay and plumbago, are very 
durable, resist sudden changes of tempe- 
rature, and may be repeatedly used, -but 
they are destroyed when saline substances 
are melted in them, and suffer combustion 
when exposed, red hot to a current of 
air. 
When placed in a furnace, crucibles should 
never be set up on the bars of the grate, 
but always upon a support. Dr. Kennedy 
found the hottest part of a furnace to be 
about an inch above the grate. They may 
be covered, to prevent the fuel or ashes 
from falling into them, with a lid of the 
same materials, or with another crucible 
inverted over them. When the fusion is 
completed, the substance may be either 
permitted to cool in the crucible, or may 
