PHARMACY. 
holes, so that the evaporation of the men- 
strmim may be prevented as much as pos- 
sible, without risk of bursting the vessel, 
fhe vessel may be heated, either by means 
of the sun’s rays, of a common fire, or of 
tlie sand-bath: and when the last is em- 
ployed, the vessel should not be sunk 
deeper in the sand than the portion tliat is 
filled. Sometimes when the menstruum 
employed is valuable, a distilling apparatus 
used to prevent any waste of it. At other 
times, a blind capital is luted on the ma- 
trass, or a smaller matrass is inverted with- 
in a larger one ; and as the vapour which 
arises is condensed in it, and runs back into 
the larger, the process in this form has got 
the name of circulation, upon which we 
have observed already. 
Decoction is performed by subjecting the 
substances operated on to a degree of heat 
which is sufficient to convert the menstruum 
into vapour, and can only be employed with 
advantage for extracting principles which 
are not volatile, and from substances whose 
texture is so dense and compact as to resist 
the less active methods of solution. When 
the menstruum is valuable, that portion of 
it which is converted into vapour is gene- 
rally saved by condensing it in a distilling 
apparatus. 
Solutions in alcohol if coloured are termed 
tinctures, and in vinegar or wine, medicated 
vinegars or wines. The solution of metals 
in mercury is termed amalgamation. The 
combinations of other metals with each other 
form alloys. 
Absorption is the condensation of a gas 
into a fluid or solid form, in consequence of 
its combination with a fluid or solid. It is 
facilitated by increase of surface and agita- 
tion ; and the power of absorption in fluids 
is much increased by compression and dimi- 
nution of temperature, although in every 
instance it be limited and determinate. Dr. 
Nooth invented an ingenious apparatus for 
combining gases wiUi flifids, and Messrs. 
Schweppe, Paul, and Cuthbertson have very 
advantageously employed compression. 
Fluids often become solid by entering 
into combination with solids, and this change 
is always accompanied by considerable in- 
crease of temperature, as in the slaking of 
lime. ^ • 
Chemical Decomposition is the separation 
of the elementary parts of bodies which 
were chemically combined: and can only 
be effected by the agency of substances 
possessing a stronger affinity for one or 
more of the constituents of the compound 
than these possess for each other. ’ 
Decomposition has acquired various ap- 
pellations, according to the phenomena 
which accompany it. 
Dissolution differs from solution in being 
accompanied by a decomposition, or change 
in the nature of the substance dissolved 
Thus, we correctly say, a solution of lime in 
muriatic acid, and a dissolution of chalk in 
muriatic acid. 
Sometimes a gas is separated during the 
action of bodies on each other. When this 
escapes with considerable violence and agi- 
tation of tlie fluid it is termed efffervescence. 
The gas is very frequently allowed to escape 
into the atmosphere, but at other times is 
either collected in a pneumatic apparatus, 
or made to enter into some new combina- 
tion. The vessels in which an effervescing 
mixture is made, should be high and suffi- 
ciently lar^e, to prevent any loss of the ma- 
terials from tlieir running over, and in soine 
cases the mjxture must be made slowly and 
gradually. ■ 
Precipitation is the reverse of solution. 
It comprehends all those processes in which 
a solid is obtained by the decomposition of 
a solution. The substance separated is 
termed a precipitate, if it sink to the bot- 
tom of the fluid ; or a cream, if it swim 
above it. Precipitation, like solution, is 
performed either vih humidd, or vid siccd ■ 
and is effected by lessening the quantity of 
the solvent by evaporation ; by diminishing 
Its powers, as by reduction of temperature 
or dilution ; or- by the addition of some che. 
mical agent, which, from its more powerful 
affinities, either combines with the solvent 
and precipitates tlie solvend, or forms itself 
an insoluble compound with some consti- 
tuent of the solution. 
The objects of precipitation are, the se- 
paration of substances from solutions in 
which they are contained; the purification 
of solutions from precipitable impurities- 
or the formation of new combinations. 
The two first means of precipitation have 
been already noticed. In performing it in 
the last manner we may observe the follow- 
ing rules: — The solution and precipitant 
must possess the requisite degree of purity. 
The solution should be perfectly saturated, 
to avoid unnecessary expenditure of the 
solvent or precipitant. The one is to be 
added slowly and gradually to the other. 
After each addition, they are to be thorough- 
ly mixed by agitation. We must allow the 
