PHARMACY. 
capable of dissolving all the active prin- 
ciples except resin, and can have its solvent 
powers assisted by a considerable degree 
of heat. 
Watery extracts are prepared by boiling 
the subject in water, and evaporating the 
strained decoction to a thick consistence. 
It is indifferent, with regard to the medi- 
cine, whether the subject be used fresh or 
dry ; since nothing that can be preserved 
in this process will be lost by drying. With 
regard to tiie facility of extraction, there is 
a very considerable difference; vegetables 
in general giving out their virtues more 
readily when moderately dried than when 
fresh. 
Very compact dry substances should be 
reduced into exceedingly small parts, pre- 
vious to the affusion of the menstruum. 
The quantity of water ought to be no 
greater than is necessary for extracting the 
virtues of the subject. This point, however, 
is not very easily ascertained ; for although 
some of the common principles of extracts be 
soluble in a very small proportion of water, 
there are others, such as the tannin, of which 
water can dissolve only a certain proportion, 
and cannot be made to take up more by 
any length of boiling, and we have no very 
good method of knowing when we have 
used a sufficient quantity of water ; for ve- 
getable substances will continue to colour 
deeply successive portions of water boiled 
with them, long after they are yielding no- 
thing to it but colouring matter. Perhaps 
one of the best methods is to boil the sub- 
ject ill successive quantities of water, as 
long" as the decoctions form a considerable 
precipitate with the test which is proper 
for detecting the substance we are extract- 
ing, such as a solution of gelatine for tannin, 
of alum for extractive, &c. 
“ The decoctions are to be depurated by 
colature ; and afterwards suffered to stand 
for a day or two, when a considerable quan- 
tity of sediment is usually found at the bot- 
tom. If the liquor poured off clear be boil- 
ed down a little, and afterwards suffered to 
cool again, it w'ill deposit a fresh sediment, 
from wliicli it may be decanted before you 
jiroceed to finish the evaporation. The 
decoctions of very resinous substances do 
pot require this treatment, and are rather 
injured by it ; the resin subsiding along with 
the inactive dregsi” 
Such are the directions given ip most of 
Dr. Duncan’s editions of the New Edin- 
burgh Dispensatory, for the depuration of 
the decoctions, and we have inserted ttiein 
at full length, because, although we doubt 
very much of their propriety, our reasons 
for so doing are scarcely more than hypo- 
thetical. We would advise the decoctions 
to be evaporated after they have been fil- 
tered boiling hot, without any further de- 
puration ; because some of the most active 
principles of vegetable substances, such as 
tannin, aie much more soluble in boiling 
than in cold water, and because almost all 
of them are very quickly affected by ex- 
posure to the atmosphere. Therefore, if a 
boiling decoclion, saturated with tannin, 
be allowed to cool, the greatest part of the 
very principle on which the activity of the 
substance depends will separate to the bot- 
tom, and according to the above directions, 
will be thrown away as sediment. The same 
objection applies more strongly to allowing 
the decoction to cool, and deposit a fesh 
sediment, after it has been partially evapo- 
rated. Besides, by allowing the decep- 
tions to, stand several days before we pro- 
ceed to their evaporation, we are in fact 
allowing the active principles contained in 
the decoction to be altered by the action 
of the air, and to be converted into sub- 
stances, perhaps inactive, wdiich also are 
throw'n away as sediment. 
The evaporation is most conveniently 
performed in broad shallow vessels: the 
larger the surface of the liquor, the sooner 
will the aqueous parts exhale. This effect 
may likewise be promoted by agitation. 
When the matter begins to grow thick, 
great care ts neces.sary to prevent its burn- 
ing. This accident, almost unavoidable if 
the quantity be large, and the fire applied 
as usual under the evaporating pan, may be 
effectually prevented, by carrying on the 
inspissation, after the common manner, no 
further than the consistence of a syrup, when 
the matter is to be poured into shallow tin 
or earthen pans, and placed in an oven, 
with its door open, moderately heated ; 
which acting uniformly on every part of 
the liquid, will soon reduce it to any de- 
gree of consistence required. This may 
likewise be done, and more securely, by 
setting the evaporating vessel in boiling 
water ; but the evaporation is in this way 
very tedious. 
Alcohol is much too expensive to be 
employed as a menstruum for obtaining ex- 
tracts, except in those cases where water is 
totally inadequate to the purpose. These 
cases are, 1. When the nature of the ex- 
tract is very perishable when dissolved in 
water, so that it is liable to be decomposed 
before the evaporation can be completed, 
especially if we cannot proceed immediate- 
