068 
PATHOLOGY. 
the fubftances, however prepared, ready for the ufe of the 
phyfician. The Materia Medica conlifts of three kinds of 
fubftances; thofe derived from animals, thofe derived 
from vegetables, and thofe taken from the mineral king¬ 
dom. Thefe divifions, as far as regards the fubftances 
themfelves, when fubjedled to the operation of chemiftry, 
run into each other. From the animal kingdom, how¬ 
ever, very few medicines are derived : the moft import¬ 
ant that occurs to us is the Lytta, or bli lie ring-fly. The 
vegetable kingdom furnifties a vaft number; and from 
every part of plants, trees, and flirubs, peculiar fubftances 
are occafionaily derivable. Our pharmacopoeias abound 
with many vegetable fubftances which are never ufed by 
practitioners in general. This fuperabundance of materi¬ 
als was owing, in the firft inftance, to an abfurd notion, 
that every individual plant in exiftencewas endowed with 
curative powers over fome particular malady; and, in 
the fecond, to that want of difcrimination which leads 
men to miftake partial coincidence for invariable or ge¬ 
neral law. The chief thing which the pharmaceutift 
has to attend to in the preparation of vegetables, is their 
prefervation. The manner of drying, therefore, whether 
in regard to its fuddennefs or tardinefs, the time of ga¬ 
thering, the mixing with other, herbs, &c. are all points 
to be intimately attended to : for fome plants, as the 
colchicum and fquill efpecially, lofe their eftefts entirely 
if not pulled at a particular feafon, or if dried in an im¬ 
proper manner. 
For the convenience of exhibition, the virtues of plants 
are extracted (for immediate ufe) by infufwn-, i. e. the 
power of cold or boiling water on a plant fora certain 
length of time ; and by decoClion, or the aClual boiling 
of the fubftance. More permanently, the virtues of 
plants are preferved in tindures, i. e. fluids containing a 
large proportion of fpirits of wine; and by exlruCls, pro¬ 
cured by means of folution and evaporation. By the 
laft procefs the effential oil is feparated from the more 
unftable conftituents of the plant. The common oils 
are feparated by exprefiion. The fubftances derived from 
the mineral kingdom are prepared by the common opera¬ 
tions of chemiftry, as precipitation, cryftallization, See. 
Hence the necefllty that the pharmacopolift Ihould be well 
verfed in the details of praClical chemiftry. The magni¬ 
ficent progrefsof this art has purged our Pharmacopoeia of 
many of its ufelefs or incongruous fubftances, and like- 
wife corrected that evil habit of preferibing which 
caufed the praftitioner to combine falts which decom- 
pofed each other. At the fame time chemiftry caufed vege¬ 
table fubftances to go very much out of ufe, at leaft 
among thofe at all influenced by the fafhion of the day. 
For, prefuming on their accurate knowledge of our fei- 
ence to interfere with the practitioner of the more diffi¬ 
cult one of medicine, fome chemilts endeavoured to ridi¬ 
cule thefe auguft bodies who had compiled the modern 
Pharmacopoeia, becaufethey had not paid fufficient atten¬ 
tion to chemical laws. In this impertinent attempt, they 
were entirely unfuccefsful, becaufe all fcientific prafti- 
tioners knew how mhch depended on thofe minute ope¬ 
rations of mineral or vegetable fubftances which elude 
the prefent knowledge of the chemift ; and they faw the 
folly of trufting to theory, and difearding a medicine 
which experience fhow'ed could cure a difeafe, merely be¬ 
caufe it was unfcientifically compounded. 
But, from whatever fource medicines are derived, much 
of theirefteft depends on their judicious combination. In 
the firft place, the practitioner muft be careful not to com¬ 
bine any minerals in a prefeription which are capable of de- 
compoling each ether ; for, though we have defended the 
uccaiional inattention to this rule in learned bodies and on 
general experience of its utility, it neverthelefs ought to be 
rigoroufly attended to in forming a prefeription. The pe¬ 
culiar affinities of the various falts are eafily learnt and 
remembered. Of the effeCi of minerals on vegetables we 
do not know much. We may remark that, generally 
fpeaking, alkalies extract more efficiently the properties 
of vegetables; on fome occafions, acids feem to have a 
flmilar effeCt; but this is uncertain. In the cafe of opium 
and fome other narcotics, the mineral acids feem to re- 
ftrain the deleterious effeCt of the drug. The analyfis of 
vegetables, as purfued by the French cheinifts, promifes to 
furnilh us at fome future period with more certain infor¬ 
mation in regard to the operation of minerals over vegeta¬ 
bles ; but at prefent we do not know enough to form any 
important general information on this head. In the com¬ 
bination of various vegetable products in the fame pre¬ 
feription, we have only to look to the medical operation of 
thefe fubftances, becaufe they have rarely any chemical 
effeCt on each other. Generally fpeaking, it is wrong to 
combine in the fame formula medicines which do not 
operate on the fame organs: thus we ffiould not combine 
diuretics and purges, nor emmenagogues and fudorifics. 
On the other hand, medicines which do aCt on the fame 
ftruCture may be combined with the utmoft advantage : 
Thus a fmall dofe of epfom falts, of fenna, and buck¬ 
thorn-juice, in combination, operates much more active¬ 
ly and certainly then a large dofe of any one of thefe 
drugs. Again; two or three diuretics mixed together 
will often operate when they have feparately failed to pro¬ 
duce any effeCt. But this combination of drugs, fo ufe- 
ful to the practitioner, does not fucceed effectually if 
puffied beyond the extent of three or four articles ; and 
it ffiould alfo be recolleCted, that, if a Ample drug does 
perform its office, it is wrong to mix it with others, be¬ 
caufe we thereby fail to improve that part of medicine we 
know fo little of; viz. the particular effeCt of particular 
drugs. 
Befides combining medicines for the fake of increafing 
their force, we fometimes combine them to reftrain 
any contingent operation of a drug, which operation is 
not effential to the cure, and is injurious to the fyftem ; 
as when we combine fulphuric acid with opium, a com¬ 
bination which does not hinder the narcotic effeCl of the 
opium, but prevents its ftimulating effeCt, which may do 
hurt; or when we give opium with calomel, and thusen- 
fure falivation while we reftrain diarrhoea. A medicine 
added to another in the above manner, for the preven¬ 
tion of any unpleafant effeCt, is called a corrective. A 
medicine which increafes the operation of anotlieris called 
an adjunCt. 
A lefs important point of confideration in preferibing 
is to render a medicine more pleafant to the palate of the 
patient, by fyrups, by acids, &c. but, in doing this, we 
muft be careful not to interfere with the operation of the 
aftive ingredients, and thus facrifice an important to a 
trifling matter. At the fame time, fo numerous are the 
formula in common ufe, that we ffiall feldom find it necef- 
fary to preferibe medicines which cannot abfolutely be 
taken; and at all events we muft avoid preferibing medicines 
which cannot be mechanically united, as acids with olea¬ 
ginous fubftances; and fo on. 
Prefcriptions are generally written in the Latin lan¬ 
guage, which of all others is the moft proper, becaufe it 
is more or lefs known to the whole world, and becaufe, 
being lefs familiar with it than with our own, we both 
read and write it with peculiar care. The quantities of 
medicine are not written in Latin, however, but are ex- 
preffed by figns. Some have ftrongly urged the propriety 
of writing the quantities as well as the names of the t 
drugs at full length; but we cannot fee that this would 
be attended with fewer miftakes than the prefent mode; 
fince the tedium of writing a long word might induce 
careleffnefs, and the figns in ufe are fumciently clear, and 
may be rapidly written in the moft correCl: manner. We 
muft except, however, the too clofe refemblance of the 
drachm and ounce figns, which cannot be defended. The 
firft fign ufed in medicine is the R, which is commonly 
fuppoled to be an abbreviation of Recipe, Take, (which 
word it is now underftood to mean in all prefcriptions ;) 
but by others is ftated to have defeended to us from thofe 
of the ancients who believed much in fidereal influence, 
and 
