but subacid fruits give a larger quantity of 
juice and of finer quality, when they are 
allowed to stand some days in a wooden or 
earthen vessel after they are bruised, 'lo 
some vegetables which are not juicy enough 
of thgmselves, the addition of a littie water 
is necessary. Lemons and oranges must 
be peeled, as their skins contain a great 
deal of essential oil, which would mix with 
the juice. The oil itself may be obtained 
separately, by expression with tlie fingers 
against a plate of glass. 
For unctuous seeds iron plates are used ; 
and it is customary not only to heat the 
plates, but to warm the bruised seeds in a 
kettle over the fire, after they have been 
sprinkled with some water, as by these 
means the product is increased, and the oil 
obtained is more limped. But as their dis- 
position to rancidity is increased by it, if 
possible this practice should be laid aside, 
or confined to exposing the bruised seeds, 
inclosed in a bag, to the steam of hot 
water. 
Despumation is generally practised on 
thick and clammy liquors, which contain 
much slimy and other impurities', not easily 
separable by filtration. The scum arises 
either by simply heating the liquor, or by 
clarifying it, which is done by mixing with 
the liquor, when cold, whites of eggs well 
beaten with a little water, which on being 
heated coagulates, and entangling the im- 
purities of the liquor, rises with them to the 
surface, and may be easily removed by a 
perforated ladle ; or the liquor may now 
be filtered with ease. Spirituous liquors 
are clarified by means of isinglass dissolved 
in water, or any albuminous fluid, such as 
milk, which coagulates by the action of 
alcohol without the assistance of heat. 
Some expressed juices, such as those of the 
antiscorbutic plants, are instantly clarified 
by the addition of vegetable acid, such as 
the juice of bitter oranges. 
Fluids can only be separated lioin each 
other when they have no tendency to com- 
bine, and when they differ in specific gra- 
vity. The separation may be effected by 
skimming off the lighter fluid with a silver 
or glass spoon; or by drawing it off by a 
syringe or syphon ; or by means of a glass 
separatory, which is an instrument having a 
projecting tube, terminating in a very slen- 
der point, through which the heavier fluid 
alone is permitted to run ; or by means of 
the capillary attraction of a spongy woollen 
thread ; for no fluid will enter a substance 
whose pores are filled by another, for which 
it has no attraction ; and, lastly, upon 
tlie same principle, by means of a filter of 
unsized paper, previously soaked in one of 
the fluids, which in this way readily passes 
through it, while the other remains behind. 
Mechanical mixture is performed by agi- 
tation, trituration, or kneading ; but these 
will be best considered in treating of the 
forms in which medicines are exhibited. 
Chemical Operatinns and Results. 
Under this chapter we have to consider 
the apparatus employed, the changes pro- 
duced, and the general analyses that ensue. 
The apparatus consists of vessels, fuel, or 
heat ; and the different modes by which 
such fuel or heat is applied, whether lamps, 
furnaces, &c. 
The vessels must necessarily vary in their 
form and materials ; upon tlie first of which 
it will be more convenient to enlarge as we 
proceed to treat of the particular operations 
in which they are employed. In choosing 
tlie materials for the construction of our 
vessels, the properties most generally re- 
quired are a power of resisting chemical 
agents, transparency, compactness, strength, 
fixity, and infusibility, and an ability to sus- 
tain sudden variations of temperature with- 
out breaking. 
Generally speaking, metals possess the 
four last properties in considerable perfec- 
tion ; but they are all opaque. Iron and 
copper are apt to be corroded by chemical 
agents ; and a solution of the last is often 
followed by dangerous affections. Tinning 
them will sometimes, but not always, an- 
swer ; for tin and lead are often too fusible. 
Platinum, gold, and silver, resist most of 
the chemical agents, but are too expensive 
for general use. 
Good earthenware resists the greatest 
intensity of heat, but has no other property 
to recommend it. Clay, the basis of all 
such wares, is plastic when worked with 
water, and sufficiently hard when burnt 
with an intense heat. But intense heat 
contracts it unduly, and it is apt to split 
and crack u|)on exposure to sudden changes 
of temperature ; whence it is necessary to 
counteract this property by the addition of 
some other substance. Siliceous sand, clay 
reduced to powder, and then burnt with a 
very intense heat, and plumbago, are occa- 
sionally used. These additions, however, 
are attended with other inconveniences ; 
plumbago especially is liable to combustion, 
and sand dimini.shes the compactness ; so 
that when not glazed tliey are porous, and 
