PER 
V E It 
r e it 
3SS 
which you observe with your eye, especially 
in objects lying near; but, according as the 
distance grows greater and greater, so the co- 
lours must be iainter and fainter, till at last 
they lose themselves in a darkish sky-colour. 
10. r i he catoptrics are best seen in a common 
looking-glass, or other polished matter, where, 
it the glass is exactly flat, the object is ex- 
actly like its original : hut, il the glass is not 
flat, the resemblance alters from the original; 
mid that more or less, according as the glass 
differs from an exact plane. 17. In drawing 
catoptric iigures, the surface of the glass is to 
he considered, upon which you mean to have 
the reflection: lor which you must make- a 
particular ichnographica! draught or pro- 
jection, which bn the glass must appear to he 
a plane full of squares ; on which projection 
transfer what shall he drawn on a plane, di- 
vided into the same number of like squares, 
where though the draught may appear very 
confused, yet the reflection of it on the glass 
will he very regular, proportional, and re- 
gularly composed, 18. The dioptric, or 
broken beam, may be seen in a tube through 
a crystal or glass which has its surface cut 
into many others, whereby the rays of the ob- 
ject are broken. For to the flat of the crystal, 
or water, the rays run straight ; but then they 
break and make an angle, which also by the 
refracted beams is made and continued on the 
other side of the same flat. 19. When these 
faces on a crystal are returned towards a plane 
placed directly before it, they separate them- 
selves at a good distance on the plane, be- 
cause they are all directed to various far-dis- 
tant places of the same. 
I Perspective plan e, is the glassor other 
transparent surface, P V, (Plate Perspective, 
fig. 2.) supposed to be placed between the eye 
and the object, perpendicularly to the horizon. 
It is sometimes called the section, table, or 
glass. 
PERSPIRATION. There seems to he 
something thrown out from the blood during 
its circulation in the arteries, at least through 
those vessels which are neat the surface of the 
body : for it is a fact, that certain substances 
are constantly emitted from the skins of ani- 
mals. These substances are known in general 
by the name of perspirable matter, or per- 
spiration. They have a great resemblance to 
what is emitted in the lungs; which renders 
it probable that both excretions are owing to 
the same cause, namely, to the decomposition 
produced in the blood by the effects of respi- 
, ration. Many experiments have been made 
to ascertain the quantity of matter perspired 
through the skin. For the first set, and not 
the least remarkable, we are indebted to 
Sancton us, who continued them for no less 
than thirty years. He ascertained his own 
w eight, and vhe weight of his food; and what- 
ever weight he lost over and above that of 
his excrements, he ascribed to perspiration. 
A similar set of experiments was afterwards 
made in France by Dodart; in England by 
Keil; in Ireland by Bryan Robertson and 
live; and in Carolina by Lining. The re- 
sult of all these experiments has been col- 
lected by Haller ; but it gives us no precise 
estimate of the amount of the transpiration, 
since these philosophers have not distinguish- 
ed between what is lost by the skin and by the 
lungs. Lavoisier and Seguin alone have at- 
tempted to ascertain the amount of the mat- 
ter perspired through the skin. A bag com- 
posed of varnished silk, and perfectly air- 
tight, wai-s procured, within which Seguin, 
who was usually the subject of experiment, 
was enclosed, and the bag was closed exactly 
over his bead. There was a slit in the bag 
opposite ito his mouth, and the edges of this 
slit wer: accurately cemented round the 
mouth by means ot a mixture of turpentine 
and pitch- Thus every thing emitted by the 
body was retained in the bag, except what 
made its escape from the lungs by respiration. 
By weighing himself in a delicate balance at 
tlie commencement of the experiment, and 
again after he had continued for some time in 
the bag, the quantity of matter carried off by 
respiration was ascertained. By weighing 
himself without this varnished covering, and 
repeating the operation alter the same inter- 
val of time had elapsed as in the former ex- 
periment, he ascertained the loss of weight 
occasioned by perspiration and respiration. 
By subtracting from this sum the loss of 
weight indicated by the lirst experiment, he 
obtained the quantity of matter which made 
its escape by perspiration in a given time. 
The following facts were ascertained by these 
experiments: 1. The maximum of matter 
perspired in a minute amounted to 26.25 
grains troy; the minimum to 9 grains: which 
gives 17.63 grains at a medium in the mi- 
nute, or 52.82 ounces in the 24 hours. This 
quantity differs less than might have been ex- 
pected from the result of former experiments 
made by Dodart, Keil, Rye, &c. 2. The 
quantity perspired is increased by drink, but 
not by solid food. 3. Perspiration is at its 
minimum immediately after a repast. It 
reaches its maximum during digestion. See 
Digestion. 
The quantity of matter perspired differs 
very considerably according to circumstances. 
It has been shewn to be greatest in hot wea- 
; ther, and in hot climates, and after great ex- 
ercise; and its relation to the quantity of 
urine has been long known. When the mat- 
ter perspired is great, the quantity of urine is 
small, and vice versa. 
To ascertain the substance thus emitted by 
perspiration is a difficult task, because it 
passes oil invisibly, and in small quantities at 
a time. It has, notwithstanding, been ascer- 
tained that water, .carbon, and an oily matter, 
are emitted ; ancl that an acid supposed to lie 
the phosphoric, phosphat of lime, and even 
urea, are sometimes emitted through the 
skin. 
1 The most accurate experiments on this 
matter that have been made are those of Mr. 
Cruikshank. He put his hand into a glass 
vessel, and luted its mouth at his wrist by 
means ot a bladder. The interior surface of 
the vessel became gradually dim, and drops 
of water trickled down. By keeping his 
hand in this manner for an hour, he collected 
thirty grains of a liquid, which possessed all 
the properties of pure water. On repeating 
the same experiment at nine in the evening 
(thermometer 62°), he collected only 12 
grains. The mean of these is 21 grains. 
But as the hand is more exposed than the 
trunk of the body, it is reasonable to suppose 
that the perspiration from it is greater than 
that from the hand. Let us therefore take 
30 grains per hour as the mean; and let us 
suppose, with Mr. Cruikshank, that the hand 
is -J-th of the surface of the body. The 
8 
perspiration in an hour would amount to 1880 
grains, and in 24 hours to 43,200 grains, or 
7 pounds 6 ounces troy. This is a'most 
double the quantity ascertained by Lavoi- 
sier and Seguin. lienee we may conclude 
that more matter is perspired through the 
hand than the other parts of the body, pn - 
vided Mr. Cruikshank’s estimate of the ratio 
between the surface of the hand and body is 
not erroneous. 
He repeated the experiment again after 
hard exercise, and collected in an hour 48 
grains of water. He found also, that this 
aqueous vapour pervaded his stocking without 
difficulty; and that it made its way through 
a shamoy-leather glove, and even through a 
leather boot, though in a much smaller quan- 
tity than when the leg wanted that covering. 
It is not difficult to see why the quantity of 
watery vapour diminishes with cold. When 
the surface of the body is exposed to a cold 
temperature, the capacity of the cutaneous 
vessels diminishes, and consequently the 
quantity which flows through them must de- 
crease. 
When the temperature, on the other hand, 
is much increased, either by being exposed to 
a hot atmosphere, or by violent exercise, the 
perspired vapour not only increases in quan- 
tity, but even appears in a liquid form. This 
is known by the name of sweat. In what 
manner sweat is produced, is not at present 
known; but we can see a very important 
service which it performs to the animal. No 
sooner is it thrown upon the surface of the 
skin than it begins to evaporate. But the 
change into vapour requires heat ; according- 
ly a quantity of heat is absorbed, and the 
temperature of the animal is lowered. This 
is the reason that animals can endure to re- 
main for some time in a much higher tempe- 
rature without injury, than could have been 
supposed. 
The experiments of Tillet, and the still 
more decisive experiments of Fordyce and 
his associates, are well known. These gen- 
tlemen remained a considerable time in a 
temperature exceeding the boiling-point of 
water. 
2. Besides water, it cannot be doubted 
that carbon is also emitted from the skin ; but 
in what state, the experiments hitherto made 
do not enable us to decide. Mr. Cruikshank 
found that the air of the glass vessel in which 
his hand and foot had been confined for an 
hour contained carbonic acid gas ; for a can- 
dle burned dimly in it, and it rendered lime- 
water turbid. And Mr. Jurine found that air 
which had remained for some time in contact 
with the skin, consisted almost entirely of 
carbonic acid gas. The same conclusion 
may be drawn from the experiments of ln- 
genhousz and Milly. Trousset has lately ob- 
served that air was separated copiously from 
a patient of his while bathing. 
Now it is evident that the carbonic acid 
gas which appeared during Mr. Cruikshank’s 
experiment, did not previously exist in the 
glass vessel; consequently it must have either 
been transmitted ready-formed through the 
skin, or formed during the experiment by the 
absorption of oxygen gas, and the consequent 
emission of carbonic acid gas. The experi- 
ments of Mr. Jurine do not allow us to sup- 
pose the first of these to be true ; for he: 
found that the quantity of air allowed to ro- 
