384 
PER 
PET 
PER 
inain in contact with the skin did not increase. 
Consequently the appearance of the carbonic 
acid gas must be owing either to the emission 
at carbon, which forms carbonic acid gas by 
combining with the oxygen gas of the air, or 
to the absorption of oxygen gas, and the sub- 
sequent emission of carbonic acid gas; pre- 
cisely in the same manner, and for the same 
reason, that these substances are emitted by 
the lungs. The last is the more probable 
opinion; but the experiments hitherto made 
do not enable us to decide. 
3. Besides water and carbon, or carbonic 
acid gas, the skin emits also a particular odo- 
rous substance. That every animal has a 
peculiar smell, is well known : the dog can 
discover his master, and even trace him to a 
distance by the scent. A dog, chained some 
hours after his master had set out on a jour- 
ney of some hundred miles, followed his foot- 
steps by the smell, and found him on the 
third day in the midst of a crowd. But it is 
needless to multiply instances of this fact ; 
they are too well known to every one. Now 
this smell must be owing to some peculiar 
matter which is constantly emitted ; and this 
matter must differ somewhat either in quan- 
tity or some other property, as we see that 
the dbg easily distinguishes the individual by 
means of it. Mr. Cruikshank has made it 
probable that this matter is an oily substance ; 
or at least that there is an oily substance 
emitted by the skin. He wore repeatedly, 
night and day for a month, the same vest of 
fleecy hosiery during the hottest part of the 
summer. At the end of this time he always 
found an oily substance accumulated in con- 
siderable masses on the nap of the inner sur- 
face of the vest, in the form of black tears. 
'When rubbed on paper, it makes it transpa- 
rent, and hardens on it like grease. It burns 
with a white flame, and leaves behind it a 
charry residuum. 
4. Berthollet has observed the perspiration 
acid; and lie has concluded that the acid 
which is present is the phosphoric: but that 
has not been proved. Fourcroy and Vau- 
queiin have ascertained that the scurf which 
collects upon the skins of horses consists 
chiefly of phosphat of lime, and urea is even 
sometimes mixed with it. It is well known 
that the sweat has a salt taste ; but hitherto it 
has not been analysed, though it probably 
differs from the transpiration. 
It has been supposed that the skin has the 
property of absorbing moisture from the air; 
but this opinion has not been confirmed by 
experiments, but rather the contrary. 
The chief arguments in favour of the ab- 
sorption of the skin, have been drawn from 
the quantity of moisture discharged by urine 
being, in some cases, not only greater than 
the whole drink of the patient, but even the 
whole of his drink and food. But it ought to 
be remembered that, in diabetes, the disease 
here alluded to, the weight of the body is 
continually diminishing, and therefore part of 
it must be constantly thrown off. Besides, 
it is scarcely possible in that disease to get an 
accurate account of the food swallowed by 
the patients ; and in those cases where very 
accurate accounts have been kept, and where 
deception was not so much practised, the 
urine was found to exceed the quantity of 
drink. In a case of diabetes, related with 
much accuracy by Dr. Gerard, the patient 
was bathed regularly during the early part of 
the disease in warm water, and afterwards in 
cold water : he was weighed before and after 
bathing, ajid no sensible difference was ever 
found in his weight. Consequently, in that 
case, the quantity absorbed, if any, must have 
been very small. 
It is well known that thirst is much allevi- 
ated by cold bathing. By this plan captain 
Bligh kept his men cool and in good health 
during their very extraordinary voyage 
across the South Sea. This has been "con- 
sidered as owing to the absorption of wafer by 
the skin. But Dr. Currie had a patient who 
was wasting fast for want of nourishment, a 
tumour in the oesophagus preventing the pos- 
sibility of taking food, and whose thirst was 
always alleviated by bathing; yet no sensible 
increase of weight, but rather the contrary, 
was perceived alter bathing. It does not ap- 
pear then, that in either of these cases water 
was absorbed. The allaying of thirst by the 
cold bathing may indeed easily be accounted 
for, by the lessening of«the temperature, and 
the prevention of perspiration. 
Further, Seguin has shewn that the skin 
does not absorb water during bathing, by a 
still more complete experiment: he dissolved 
some mercurial salt in water, and found that 
the mercury produced no efjjpct upon a person 
that bathed in the water, provided no part of 
the cuticle was injured; but upon rubbing off 
a portion of the cuticle, the mercurial solu- 
tion was absorbed, and the effects of the mer- 
cury became evident upon the body. Hence 
it follows irresistibly, that water, at least in 
the state of water, is not absorbed by the 
skin when the body is plunged into it, unless 
the cuticle is first removed. 
1 his may perhaps be considered as a com- 
plete proof that no such thing as absorption 
is performed by the skin; and that therefore 
the appearance of carbonic acid gas, which 
takes place when air is confined around the 
skin, must be owing to tiie emission of car- 
bon. But it ought to be considered, that al- 
though the skin cannot absorb water, this is 
no proof that it cannot absorb other sub- 
stances ; particularly that it cannot absorb 
oxygen gas, which is very different from wa- 
ter. It is well known that water will not 
pass through bladders, at least for some time: 
yet Dr. Priestley found that venous blood 
acquired the colour of arterial blood from 
oxygen gas, as readily when these substances 
were separated by a bladder, as when they 
were in actual contact. He found, too, that 
when gases were confined in bladders, they 
gradually lost their properties, it is clear 
from these tacts, that oxygen gas can per- 
vade bladders ; and if it can pervade them, 
why may it not also pervade the cuticle? 
Nay, further, we know from the experiments 
of Cruikshank, that the vapour perspired 
passes through leather, even when pre- 
pared so as to keep out moisture, at least for 
a certain time. It is possible, then, that wa- 
ter, when in the state of vapour, or when dis- 
solved in air, may be absorbed, although wa- 
ter, while, in the state of water, may be inca- 
pable of pervading the cuticle. The experi- 
ments, therefore, which have hitherto been 
made upon the absorption of the skin, are 
insufficient .o prove that air and vapour can- 
not pei\ade the cuticle, provided there are 
any facts to render the contrary supposition 
Now that there are such facts, cannot ho 
denied. We shall not indeed produce the 
experiment of Van Mons as a fact of that 
kind, because it is liable to objections, and at 
best is very indeci -ive. Having a patient 
under his care who, from a wound in the 
throat, was incapable for several days of tak- 
ing any nourishment, lie kept him alive -dur- 
ing that time, by applying to the skin in dif- 
ferent parts of the body, several times a dav, 
a sponge dipt in wine or strong soup. ‘A 
fact mentioned by Dr. V atson is much more 
important, and much more decisive. A l;ul 
at Newmarket, who had been almost starved 
in order to bring him down to such a weight 
as would qualify him for running a horse- 
race, was weighed in the morning of the race- 
day; lie was weighed again an hour after, 
anti was found to have gained 30 ounces of 
weight; yet in the interval he had only taken 
half a glass of wine. Here absorption must 
have taken place, either by the skin, or lun^s, 
or both. The difficulties in either case are 
the same; and whatever renders absorption 
by one probable, will equally strengthen the 
probability that absorption takes place by the 
other. See Physiology. 
FERULA, a genus of the class and order 
dioecia polyandria. 1 here is one species, a 
tree of New Grenada. 
PETAL, among botanists, an appellation 
given to the flower-leaves, in opposition to 
the folia, or common leaves of the plant. 
See Botany. 
PETALOMA, a genus of the decandria 
monogynia class and order: the calyx is 
goblet-shaped, five-toothed ; petals five; sta- 
mina on margin of eaiyx; berry one-celled, 
seeds one or four. There are two species* 
trees of Jamaica and Guiana. 
PE I ARD, ii: the art of war, a metalline 
engine, somewhat resembling a high-crowned 
hat. 4 he petard may be considered as a 
piece of ordnance ; it is made of copper mixed 
with brass, or ot lead with tin: ils charge is 
from five to six pounds of powder, which 
reaches to within three iingers’-breadth of the 
mouth ; the vacancy is filled with tqw, and 
stopped with a wooden tompion, the mouth 
being strongly bound up with cloth tied very 
tight with ropes. It is covered up with a 
madrier, or wooden plank, that has a cavity 
to receive the mouth of the petard, and fast- 
ened down with ropes. 
Its use is in a clandestine attack to break 
down gates, bridges, barriers, &cc. to which it * 
is hung ; and this it does by means of the 
wooden plank. It is also used in counter- 
mines to break through (he enemy’s md- 
leries, and give their mines vent. ‘The* 3 in- 
vention of petards is ascribed to the French 
Huguenots, in 1 579* who with them took Ca» 
hors, as d’Aubigne tells us. 
PETECI1LE. See Medicine. 
P Ed E R -PEN C E, an antient tax of a 
penny on each house, paid to the pope. It 
w as called Peter-pence, because collected on 
the day of St. Peter ad vincula, and sent to 
Rome; whence it was also called llome-scot, 
and Rome-penny. 
PETESIA, a genus of the tetrandria mo- 
nogynia class and order: the corolla is one- 
petalled, funnel-form, stigma bifid, berry 
many-seeded. There are three species, shrubs 
of South America and. the West Indies, 
