5. In consequence of the chemical action_A Treatise on the Plague and Yellow Fever With an Appendix, containing histories of the plague at Athens in the time of the Peloponnesian War; at Constantinople in the time of Justinian; at London in 1665; at Marseilles in 1720

by which it is expelled out of the body will be different; for, as

all medicines are extraneous substances, they must be sent out of the

body as fast as possible; and it is their action upon one particular

part which promotes their expulsion. Thus, if from the nature of the

medicine it acts in a certain way upon the stomach and bowels, it will

vomit or purge, or perhaps both; and by this action it is expelled

from the body, along with whatever other matters happen to be in the

stomach or intestines; and thus medicines do good only accidentally;

for mere vomiting or purging are most certainly diseases; but where

noxious matters exist in the bowels, and do not naturally excite

these operations, an emetic or purgative is unquestionably useful.

Here the authors of the Science of Life reason differently; and it is

worth while to refute their argument, as being the foundation of such

_tremendous_ practice as nobody of common sense would choose to be the

subject of. Of tartar emetic they speak in the following terms. “That

tartar emetic is a stimulant of very high power, is evident from the

small quantity of it which produces the state of indirect debility that

occasions vomiting. It should be given in such a manner as to increase

and to support the excitement. But this will be found difficult, as the

duration of its action seems to be even shorter than that of opium.

If its action does not continue more than a quarter of an hour, might

it not be repeated at such short intervals, and the doses so reduced

as to allow the establishment of the indirect debility?” This is

arguing in a circle. They first suppose that vomiting is occasioned

by indirect debility, that is, the weakness produced by an excessive

stimulus to the whole system, as in cases of drunkenness; and then,

from the existence of vomiting, they prove that a general stimulus

had pre-existed. The cases, however, are widely different. In cases

of drunkenness, the person feels himself at first exhilarated, alert

and active, which shows the existence of a general stimulus. But

who has ever found himself exhilarated by taking a dose of tartar

emetic? Yet in a general excitement it is absolutely necessary that

this exhilaration should take place, because it is an inseparable

consequence of an addition of vital power, let it come in what way

it will. Thus we know that if a person happens to be much exhausted

by fatigue and abstinence, he will be exhilarated and his strength

augmented by a single mouthful of meat, as well as by a glass of wine.

This shows that both these are general stimulants to the system; but

what medicine have we that will produce similar effects? Perhaps opium

comes the nearest in the whole materia medica; but the uneasiness

it occasions in the stomach manifests a greater action upon it than

the other parts; for if the whole body were equally excited, the

withdrawing of the stimulus, or its naturally losing its force, could

only have the same effect with fasting or fatigue; but the debility of

the stomach, the confusion of the head, and other effects which attend

a dose of opium, demonstrate that it acts partially, and not equally

over the whole body. The Science of Life indeed says that these effects

are owing to the improper omission of the medicine, or not repeating

the doses in due time. This may be; but no improper exhibition of

food, or want of due repetition, will produce such symptoms; which

undoubtedly is a proof that food stimulates the system in one way, and

opium in another.

6. If any medicine could be found that acted as an universal stimulus

or exciter of the whole system, it could not like others be expelled,

by any particular evacuation; but, by destroying the balance between

the force of the acting powers and the subject on which they act,

would most certainly kill, unless very powerful means were used to

counteract its effect. The only stimulant we are acquainted with which

acts equally on the whole system, and which can be readily exhibited

as a medicine, is that pure kind of air called by Dr. Priestley

_dephlogisticated_, by Scheele _empyrean_, and by the French chemists

and their followers _oxygen_. The exciting powers of this air, when

breathed instead of the ordinary atmosphere, are astonishing. It not

only augments the appetite, but the power of the muscles, and the

inclination to use them; so that without any intoxication or delirium

the person cannot refrain from action; and it not only exhilarates

the spirits in an extraordinary manner, but beautifies the face. Did

the cure of diseases therefore, or any set of them, depend on mere

excitation, no other medicine but _oxygen_ would be necessary. What

effects it may have in diseases of debility is not yet ascertained; but

to persons in health it certainly proves fatal: their bodies are unable

to bear its powerful action, and of consequence they waste, and would

die of consumptions, if its effects were not counteracted. Nor is this

at all an easy matter; for Dr. Beddoes informs us that, by breathing

this air for a short time each day, only for three weeks, he found

himself in great danger of a consumption, and was obliged to use much

butter and fat meat in his diet, besides giving up the use of the air

altogether, in order to get clear of its mischievous effects.

Another mode of stimulating or exciting the whole system is, by

putting into it a larger quantity of blood than it naturally contains.

This is entirely similar to the breathing of _oxygen_; especially if

arterial blood be used, which has already imbibed its spiritous part

from the atmosphere. In the last century the transfusion of blood was

proposed not only as a mode of curing diseases, but of restoring old

people to youth; and Dr. McKenzie, in his Treatise on Health, quotes

from the memoirs of the Academy of Sciences several instances of the

blood of brute animals being infused into human veins, without any

inconvenience. It seems, however, not only a bold but an unnatural

attempt to use the blood of beasts for such a purpose; and, however

lavish mankind may be of their blood upon certain occasions, it is

to be feared that there are few who would be willing to spare any to

relieve another from sickness; but indeed little can be said about the

practice; as, on account of some bad consequences, or failures, it was

forbidden by the king of France, and by the pope’s mandate in Italy,

and has now fallen into disuse. In a paragraph at the end of Heister’s

surgery (4to edition) it is asserted that the transfusion of blood was

productive of madness. Dr. Darwin, however, in his Zoonomia, still

proposes the transfusion of blood as a remedy, and even describes a

convenient apparatus for performing the operation. In one part of his

work he says, “Might not the transfusion of blood, suppose of four

ounces daily, from a strong man, or other healthful animal, as a sheep

or an ass, be used in the early state of nervous or putrid fevers?”

In another place he mentions his having proposed it to a gentleman

whose throat was entirely closed up by an incurable swelling, so that

he could swallow nothing. This is a disease not very rare, and which

always must be fatal; because the patients, though not affected by

any sickness, die of hunger; and, to relieve them from this miserable

situation, extraordinary attempts are not only allowable but laudable.

The Doctor proposed to his patient, “to supply him daily with a few

ounces of blood taken from an ass, or from the _human animal_, who is

_still more patient and tractable_, in the following manner: To fix a

silver pipe, about an inch long, to each extremity of a chicken’s gut,

the part between the two silver ends to be measured by filling it with

warm water; to put one end into the person hired for that purpose, so

as to receive the blood returning from the extremity; and when the gut

was quite full, and the blood running through the other silver end,

to introduce that end into the vein of the patient, upwards towards

the heart, so as to admit no air along with the blood. And, lastly, to

support the gut and silver ends on a water plate filled with water of

98 degrees of heat; and, to measure how many ounces of blood were taken

away, to compress the gut from the receiving pipe to the delivering

pipe.” The gentleman desired a day to consider of this proposal, and

then another; after which he totally refused it, saying that he was now

too old to have much enjoyment of life, and that, being so far advanced

in a journey which he must certainly accomplish sooner or later, he

thought it better to proceed than return. The Doctor informs us that

he died a few days afterwards, seemingly very easy, and careless about

the matter. One experiment of this kind I have been witness to; not

indeed on a human creature, but on a calf. This creature received into

one of its jugular veins a considerable quantity of blood from the

carotid artery of another, nearly of the same age (about a month, or

little more.) It was impossible to say any thing about how much was

transfused; only the bleeding was continued till the animal which lost

the blood began to shew signs of faintness. The artery was then tied

up, and the orifice in the jugular vein closed. The calf which had

lost the blood appeared very languid and faint, but lived a few days

in a drooping state; when it either died of itself, or was killed,

as being supposed past recovery. The other, which had received the

blood, appeared to be in every respect highly excited. It became

playful, even in the room where the operation was performed, its eyes

assumed a bright and shining appearance, and its appetite was greatly

increased. Thus it continued for about a fortnight; appearing all the

time to be in high health, and eating much more than usual; but at last

died suddenly in the night. From these effects on healthy subjects,

however, we cannot infer what would happen in such as are diseased;

but it is plain that if the cure of diseases were to depend upon mere

_excitation_, the means are in our power, without any local irritation,

which always must take place in some degree by the use of ordinary

medicines. This path is not absolutely untrodden: the pneumatic

practitioners of the present day have tried oxygen in consumptions, and

found it pernicious; and Dr. McKenzie informs us that the transfusion

of blood was tried ineffectually in the same.

7. As all the medicines usually prescribed at present are only to be

accounted partially stimulant, or as acting upon particular parts of

the system, we see that some may promote one evacuation, and some

another; while all produce some change in the organization, which may

prove useful or detrimental, may increase the disease or cure it,

or may produce another, according to the judicious or injudicious

application. But for a knowledge of all this we must be indebted to

experience: there is not a theory on earth that can lead us a single

step.

Before we dismiss the consideration of medical theories, however,

it will still be necessary to give some account of the new system

as it hath branched out in various ways: for though the fundamental

principle is now received by a great number of physicians, yet the

superstructure is exceedingly different from what Dr. Brown himself

erected and, indeed, from the very same principles we find conclusions

made as directly opposite to one another as can be expressed in words.

Drs. Yates and McLean, for instance, at Calcutta in the East Indies,

have concluded that the plague “is a disease of a very high degree of

exhaustion;” which Dr. Brown would have called debility. Dr. Rush at

Philadelphia, proceeding also upon the Brunonian principles, determines

it to be the most inflammatory of all diseases,[62] and which Dr. Brown

would have called a disease of excitement. These two doctrines are, in

every sense of the word, as distant from one another as east from west.

Let us then consider both, if any consideration can avail us on the

subject.

[62] See above, p. 102.

By the ancients it was supposed that diseases were occasioned by

something either bred in the body or received into it, and that the

power of nature produced, during the course of the disease, a certain

change in this matter, called _coction_, or _concoction_; which, if we

please, we may express by the English word _cooking_. The matter of the

disease, called also _morbific_ matter, thus _cooked_, was in a state

proper for expulsion, and was therefore thrown out by sweat, vomit,

stool, &c. or it might be expelled artificially, which could not have

been attempted with safety before. Modern systems deny the existence

of morbific matter, and resolve all into an affection of the nerves,

according to Dr. Cullen by certain sedative causes, but according

to Dr. Brown by an accumulation in some cases, and an exhaustion in

others, of the excitability or excitement of the body. The Science of

Life commences with stating what they suppose to be an improvement

of the Brunonian principles, and from which the following account of

the origin of diseases is extracted. “Upon the different states of

_excitability_ depend all the phenomena of health and disease. There

are three states of the excitability. 1. The state of accumulation;

when a portion of the usual stimuli is withheld.... When a portion

of the usual stimuli is withheld, the excitability accumulates, and

the body becomes susceptible of impression in the direct ratio of

the subduction. This state constitutes diseases of accumulation,

or of direct debility. 2. The middle state; when the excitability

is such that the application of the accustomed degree of exciting

powers produces _tone_ or _health_. 3. The state of exhaustion. When

the application of stimuli has been greater than that which produces

healthy action, the excitability is exhausted, and the body becomes

less susceptible of impression in the direct ratio of the excess. This

state constitutes diseases of exhaustion, or of indirect debility. The

states of accumulation and exhaustion of the excitability, in their

different degrees, constitute all the diseases to which living bodies

are subject.”

Here the chime runs on the word _excitability_, which is not defined.

If we call this property _life_, then we are only informed, that, as

life is more or less vigorous, the body enjoys a greater or smaller

degree of health; which we know without any medical instructor. If,

instead of the accumulation and exhaustion of excitability, we take the

original doctrine of excitement and debility laid down by Dr. Brown

himself, we are nothing better. The whole theory is lost for want of

the definition of a single word. As long as _excitability_ remains an

unknown property, we can explain nothing by it. We may indeed vary

our terms. We may call it _nervous influence_ with Dr. Cullen, or

_sensorial power_ with Dr. Darwin; but we shall still be as much in the

dark as ever; and all that can be made out of our theories, when our

language is _decyphered_, must be, that sometimes people are well, and

sometimes they are sick!

Dr. Rush, in his Treatise on the Proximate Cause of Fever, adopts in

part Dr. Brown’s system pretty nearly as the author himself laid it

down. “Fevers of all kinds (says he) are preceded by general debility.

This debility is of two kinds, viz. direct and indirect. The former

depends upon an abstraction of usual and natural stimuli; the latter

upon an increase of natural, or upon the action of preternatural,

stimuli upon the body.... Debility is always succeeded by increased

excitability, or a greater aptitude to be acted upon by stimuli.... The

diminution or abstraction of one stimulus is always followed by the

increased action of others.” Here it is evident we are as much in want

of definitions as ever. We know neither what _excitability_ is, nor

what _debility_ is, and yet they are both held out as the _causes_, and

_proximate_ or _immediate_ causes, too, of symptoms produced by things

quite obvious to our senses. Thus cold and heat, with which we are

daily conversant, are only called the _predisposing_ causes of fever;

while _debility_ and _excitement_, words to which we have no meaning,

are said to be the _proximate_ cause. It would certainly be better to

throw away such words altogether, and say that cold, heat, &c. cause

fevers, without troubling ourselves farther about the matter.

It remains now to take into consideration the pneumatic theories,

founded upon the discoveries made by Dr. Black, Dr. Priestley,

Lavoisier, and others, concerning various kinds of aerial fluids, or

_gases_,[63] as they are also called. Some of these, particularly

that afterwards called fixed air, were discovered by Van Helmont.

Considerable advances were made by a German chemist, named _Mayow_, in

the last century; but his book had fallen into such oblivion that his

name was scarce ever mentioned, until his discoveries were repeated,

and still greater advances made by others. Dr. Hales obtained air from

a great many different substances, but was unable to ascertain any

thing concerning its nature. Dr. Black of Edinburgh laid the foundation

of pneumatic chemistry, by discovering that a certain species of air is

capable of being absorbed by earths of different kinds, and that many

very heavy substances owe at least one half of their weight to this

condensed air. The discovery was accidental. Wishing to obtain a very

pure and white lime, he had recourse to the fine white earth called

_magnesia alba_. Some of this he distilled with a heat sufficient to

make the vessel red hot. Only a very small quantity of water came over,

but the magnesia had lost almost two thirds of its weight. This immense

loss was found to arise from an emission of air during the operation;

and by other experiments it was likewise found that the air might be

transferred from one portion of magnesia to another from which it had

been previously expelled; that the existence of this species of air in

certain bodies was the cause of that fermentation which takes place

when any acid is poured upon them, as vinegar upon chalk or potash.

Hence if any of these substances be deprived of its air, it will not

any longer ferment in this manner. It must not be forgot, however,

that when air thus unites itself with any terrestrial substance it no

longer has its former properties. It is reduced exceedingly in bulk,

and in proportion to this reduction only the body is increased in

weight; and therefore though we say that the _air_ is absorbed, we must

still remember that only _one part_ of it is so, and that by far the

least considerable in bulk. A violent fire will always expel the air

again, and restore it to its former bulk; and again the condensation

or absorption of the air is always attended with the production of

heat. This last property was not much attended to by Dr. Black, but

others have observed it; and the late Dr. Charles Webster of Edinburgh

published a theory in which he maintained that condensation was in

_all_ cases the cause of of heat. But, however true it may be that

condensation of any kind is followed or accompanied by heat, it is

evidently necessary to know the cause of the _condensation_ also,

otherwise we make no advance in solid theory.

[63] _Gas_ is a German word, or derived from one, signifying

_spirit_. The word _ghost_ comes from the same original.

The aerial fluid, discovered by Dr. Black, was one of those most

commonly met with. He called it _fixed air_, from its property of

adhering or fixing itself to different bodies. It was found to be the

same with that which had been discovered by Van Helmont, and by him

named _gas sylvestre_ (spirit of wood)[64] or the fume of charcoal;

it was found to be the same with the steam of fermenting liquor, and

with that very frequent and dangerous vapour, met with in coal mines,

called in Scotland the _choke-damp_. Like other discoveries, this was

quickly pushed beyond its proper bounds, and applied to the solution of

phenomena which it could not solve. Dr. MBride, particularly, supposed

it to be the bond of union between the particles of matter, or in other

words the principle of cohesion itself. It was also supposed to be

the substance of those scorching winds, called _samiel_, met with in

Asia and Africa, and which sometimes prove fatal to travellers. The

pernicious vapours called _mofetes_, which sometimes issue from the old

lavas of Vesuvius in Italy, were likewise supposed to be the same;[65]

but of this, particularly with regard to the samiel, there seems to be

no sufficient evidence.

[64] This must be understood only of its general properties and

effects; for, though the fume of charcoal possesses many of the

apparent properties of pure fixed air, it contains also a very

considerable quantity of another kind of gas.

[65] Many fabulous stories have been related concerning the samiel.

Even so late a traveller as Mr. Ives has adopted some of those

exaggerated accounts which have been discredited by those who have

long resided in the countries where this wind is commonly met with.

It is not peculiar to the deserts of Arabia, but is met with in all

hot countries which are destitute of water. In the African deserts

therefore it is common; and Mr. Bruce describes it by the name of

_simoom_. It was preceded by whirlwinds of a very extraordinary

kind. “In that vast expanse of desert (says he) from W. and to N. W.

of us, we saw a number of prodigious pillars of sand at different

distances, at times moving with great celerity, at others walking on

with a majestic slowness. At intervals we thought they were coming

in a very few minutes to overwhelm us; and small quantities of sand

did actually more than once reach us. Again they would retreat so as

to be almost out of sight; their tops reaching to the very clouds.*

There the tops often separated from the bodies; and these, once

disjoined, dispersed in the air, and did not appear more. Sometimes

they were broken near the middle, as if struck with a large cannon

shot. About noon they began to advance with considerable swiftness

upon us, the wind being very strong at north. Eleven of them ranged

along side of us at about the distance of three miles. The largest

of them appeared to me at that distance to be about ten feet

diameter.... It was in vain to think of flying; the swiftest horse or

the fasted sailing ship could be of no use to carry us out of this

danger; and the full persuasion of this rivetted me as if to the spot

where I stood.” At another time he saw them in much greater number,

but of smaller size. They began immediately after sunrise, like a

thick wood, and almost darkened the sun. His rays darting through

them gave them the appearance of pillars of fire. They now approached

to the distance of two miles from our travellers. At another time

they appeared beautifully spangled with stars. in Darwin’s Botanic

Garden we find a reason assigned for the appearance of these

whirlwinds; viz. the impulse of the wind on a long ledge of broken

rocks which bound the desert. By these the currents of air which

struck their sides were bent and were thus like eddies in a stream of

water which falls against oblique obstacles. In the same work we have

the following poetical description of them:

* _N. B._ In these sandy deserts, where it never rains,

there are no clouds.

“Now o’er their heads the whizzing whirlwinds breathe,

And the live desert pants and heaves beneath;

Ting’d by the crimson sun, vast columns rise

Of eddying sands, and war amid the skies,

In red arcades the billowy plains surround,

And whirling turrets stalk along the ground.”

Whether the simoom is always preceded by these whirlwinds we know

not; but Mr. Bruce mentions an extreme redness of the air, pointed

out by his attendant Idris, as the sure presage. His advice was, that

all of them, upon the approach of the pernicious blast, should fall

upon their faces, with their mouths on the earth, and hold their

breath as long as possible, so that they might not inhale the deadly

vapour. They soon had occasion to follow this advice; for next day

Idris called out to them to fall upon their faces, for the simoom

was coming. “I saw (says Mr. Bruce) from the S. E. a haze coming, in

colour like the purple part of the rainbow, but not so compressed

or thick. It did not occupy twenty yards in breadth, and was about

twelve feet high from the ground. It was a kind of blush upon the

air, and it moved very rapidly; for I could scarce turn to fall upon

the ground, with my face to the northward, when I felt the heat of

its current plainly upon my face. We all lay flat on the ground, as

if dead, till Idris told us it was blown over. The meteor, or purple

haze, which I saw, was indeed passed; but the light air that still

blew was of heat sufficient to threaten suffocation. For my part, I

felt distinctly in my breast that I had imbibed a part of it; nor

was I free of an asthmatic sensation till I had been some months

in Italy, at the baths of Poretta, near two years afterwards.” It

continued to blow for some time, and in such a manner as entirely to

exhaust them, though scarcely sufficient to raise a leaf from the

ground.

The account given by Mr. Ives is, that it blows over the desert (of

Syria) in the months of July and August, from the northwest quarter,

and sometimes continues with all its violence to the very gates of

Bagdad but never affects any body within its walls. Some years it

does not blow at all and in others it comes six, eight, or ten times,

but seldom continues more than a few minutes at a time. It often

passes with the apparent quickness of lightning. The sign of its

approach is a thick haze, which appears like a cloud of dust rising

out of the horizon, on which they throw themselves with their faces

on the ground, as already mentioned. Camels are said, instinctively,

to bury their noses in the sand. As for the stories of its dissolving

the cohesion of the body in such a manner that a leg or an arm may

be pulled away from those who are killed by it, or that their bodies

are reduced to a gelatinous substance, we cannot by any means give

credit to them. From its extreme quickness, and luminous appearance,

it would seem to be an electrical phenomenon immediately preceding

those vehement hot winds which all travellers agree in likening to

the vapour issuing from a large oven when the bread is newly taken

out. Its electrical nature will be more probable from the account

given by Mr. Ives, that the Arabians say it always leaves behind it a

very sulphureous smell. These particulars do not at all accord with

the supposition of its consisting of fixed air. I have indeed been

assured by a gentleman long in the service of the English East India

Company, that the samiel cannot pass over a river. Hence probably it

has been supposed to be a blast of fixed air, because this species

of gas is readily absorbed by water; but we know that the same thing

would also take place with any quantity of electric matter; for water

takes up this also much more completely than it does fixed air.

The _mofetes_ are invisible, and kill in an instant. They rise from

old volcanic lavas, and, as it were, creep on the ground, and enter

into houses, so that they are very dangerous; but, though they may

probably consist of fixed air, we have not as yet any direct proof

of it. It is not indeed easy to imagine why any lava should suddenly

emit a great quantity of fixed air, and then as suddenly cease; nor

in what manner the air thus emitted should continue unmixed with

the atmosphere; for fixed air will very readily mix in this manner,

insomuch that a large quantity of it being let loose in a room has

been found to vanish entirely in less than half an hour. Sir William

Hamilton mentions a mofete having got into the palace of the king of

Naples.

The industry of other experimenters did not long leave theorists

without abundance of materials upon which they might exercise their

talents. It is impossible in this place to assign to each his proper

rank in the way of discovery, or indeed to mention their names. Dr.

Priestley has distinguished himself far above the rest. He not only

repeated and improved Dr. Black’s experiments on _fixed air_, but

likewise found out a number of other kinds; particularly that from

animal substances in a state of putrefaction, which is so pernicious

to living creatures, insects excepted; for these last will thrive

amazingly in air that would prove certain death to a man. He also

discovered that this kind of air, and some others, were absorbed by

vegetables, and thence inferred the use of vegetables in purifying

the atmosphere. He even analysed the atmosphere itself, and found

that it consisted of two different kinds of fluids, one of which he

called _dephlogisticated_, the other _phlogisticated_ air. The former

was found to support animal life for a time, the latter to destroy it

instantly. Their effects upon fire were the same; the former exciting

the most vehement heat and bright flame, the latter extinguishing a

fire at once.

The fame of Dr. Priestley’s discoveries quickly reached the continent

of Europe; the French chemists repeated his experiments with

improvements, as they thought; and indeed certainly made many curious

discoveries. Lavoisier was particularly remarkable for his numerous

and accurate experiments; but, by his changing entirely the language

of former chemists, and substituting a set of new terms of his own

invention, he certainly entailed the greatest curse upon the science

it ever met with. It belongs not to this treatise to give an account

of his system farther than to say, that, from the immense proportion

of condensed aerial matter found in most terrestrial substances, he

and his followers were led to conclude, that different species of

air constitute almost the _whole_ of the terraqueous globe. Water

particularly they have absolutely and most positively determined to

be a composition of two airs condensed, viz. the dephlogisticated and

inflammable, which they call _oxygen_ and _hydrogen_. However, this

doctrine is still opposed by Dr. Priestley and some others.

In the midst of so much theory, and so many new and surprising

discoveries, it would have been wonderful indeed if the science of

medicine had kept free from innovation. It did not: the new chemistry,

with all its formidable apparatus of hard words, was introduced,

and thus the study of the science, already very difficult, was

rendered still more so. In passing this censure upon the modern

_nomenclature_, as it is called, I am sensible that I must rank with

the minority; nevertheless, I have the satisfaction of finding that I

am not altogether singular. Dr. Ferriar, in the preface to his second

volume, complains, “that, with every attempt towards the formation

of a system, new applications of words are introduced, which, though

desirable in the art of poetry, are very inconvenient in pathological

books, especially when this is done to give an air of novelty to old

theories and observations. For, between the ancient language, which

practitioners cannot entirely reject, and the new dialect, which they

cannot wholly adopt, the style of medical books is reduced to a kind

of jargon, that the author himself may possibly understand, but which

his readers find it very difficult to unriddle. Hence results a neglect

of medical literature, and hence the pernicious habit of regarding as

new whatever has not appeared in the publications of the last half

century.” To the same or a similar purpose, in the preface to his first

volume, he cites Quintilian. [66]“Some have such a multitude of vain

words, that, while they are afraid of speaking like other people, by a

kind of affected elegance, they confound every thing they have to say

with their immense loquacity.”

[66] Est etiam in quibusdam turba inanium verborum, qui dum communem

loquendi morem reformidant, ducti specie nitoris, circumeunt omnia,

copiosa loquacitate, quæ dicere volunt.

The pneumatic system naturally arose from a consideration of the

composition of the atmosphere we breathe. Finding this fluid to be

composed of two others, the one of which would preserve life for some

time at least, and the other instantly destroy it, it became natural to

think that diseases might be produced by any considerable variation in

the proportion of these ingredients. An instrument was soon invented by

which any considerable variation in this respect might be discovered;

but upon trial this was found to be of very little use. Dr. Priestley

himself tried, by means of this instrument, some very offensive air

which had been brought from a manufactory, and could find no remarkable

difference between it and that which was accounted pure. Still,

however, it was evident that by increasing very much the proportion

of one of the ingredients, some considerable alteration might be

produced, which could not but be perceptible in the human body; and

this led to the application of aerial chemistry to disorders of the

lungs. The mixture chosen for this purpose was pure dephlogisticated

(_oxygen_) with inflammable air (_hydrogen_;) and, though this has not

been known to effect a radical cure, it certainly has given relief in

many cases. In fevers also the application of fixed air (carbonic acid)

hath been found advantageous; but with regard to oxygen and some others

we have not yet a decided instance of their good effects in any case.

Dr. Beddoes indeed is of opinion that it would be of service in the

sea-scurvy; but in this (whether his conjecture be right or wrong) the

theory is certainly erroneous, as shall presently be evinced.

In considering the pneumatic system it is evident that modern chemists

have fallen into the same error with their predecessors, viz. of

supposing that every thing which by the force of fire or otherwise they

could produce, from any substance, previously existed in it. Hence, as

from a piece of bone for instance, a chemist can produce water, salt,

oil and earth, it was supposed that these four were the principles or

elements of the bone. But this was false reasoning; for if these were

really the chemical principles, they ought to have been able to produce

some kind of bony substance by mixing them together after they had been

distilled. But no such thing could be done; and though we should add to

the mixture the whole quantity of air emitted during the distillation,

and which escaped the notice of ancient chemists, our success would

be no better. In like manner, because in certain circumstances oxygen

is obtained from the flesh of animals, it has been concluded that it

necessarily exists as an ingredient in their bodies while living; and

that, if this kind of air happens to predominate, the animal will be

affected in one way, or if hydrogen prevail, in another. But though

we have already quoted Dr. Girtanner with approbation as having

obtained oxygen gas from fresh meat, yet this does not by any means

prove to us that it exists in flesh as one of its component parts.

Even in the Doctor’s experiment it was necessary to expose the flesh

to the atmosphere in order to procure the gas by distillation; which

undoubtedly must excite a strong suspicion that the air in question

comes from the atmosphere itself; and, if this is the case, it is not

reasonable to suppose that a disease could be cured by any addition

of oxygen to the solid parts; because, though sound flesh may have an

inclination to absorb this kind of air, we do not know whether it would

have such a property of absorption in a diseased state. Indeed in the

scurvy, which Dr. Beddoes chooses as an example, experiment seems to

determine in favour of _fixed air_ rather than any other. But let us

hear Dr. Girtanner himself, who has at large discussed this subject

in two memoirs; one upon the laws of irritability, and another on the

principle of irritatibility.

In these memoirs we find the Brunonian doctrine set forth with such

silence in regard to Dr. Brown himself, that some have not scrupled to

charge Dr. Girtanner with literary _theft_; but this is a matter which

belongs not to us to consider: the theory may be very good, whether

stolen or not. He changes the word _excitability_, used by Dr. Brown,

for _irritability_; but hath the misfortune of not being able to tell

us what he means by it. He goes on, however, to distinguish the three

states of _tone_ or health, _accumulation_, and _exhaustion_, as other

Brunonians do. Health, he says, in a fibre “consists in a certain

quantity of the irritable principle necessary for its preservation. To

maintain this state, the action of the stimulus must be strong enough

to carry off from the fibre the surplus of this irritable principle

which the lungs and the circulation of the fluids are continually

supplying. For this a certain equilibrium is necessary between the

stimuli applied and the irritability of the fibre, in fine that the sum

of all the stimuli acting upon it may be always nearly equal; powerful

enough to carry off from the fibre the excess of its irritability, and

not so strong as to carry off more than this excess.... When the sum

of the stimuli acting upon the fibre is not great enough to carry off

all its excess of irritability, the irritable principle accumulates in

the fibre, and then it is found in that state which I call the _state

of accumulation_; the irritable principle accumulates in the fibre,

its irritability is augmented, and the stimuli produce much stronger

contractions than when the fibre only retains its tone.... When the

sum of the stimuli acting upon the fibre is too great, the fibre is

deprived not only of the excess of its irritability, but also of some

portion of the irritable principle necessary for the tone of the fibre;

or, more properly speaking, the fibre loses more irritability than

it receives, and, of course, in a short time finds itself in a state

of _exhaustion_; and this exhaustion will be either _temporary_, or

_irreparable_.”

Here it is evident that we have nothing but Dr. Brown’s system, without

the least explanation to render it more intelligible. A definition

is still wanting. This invisible and incomprehensible property of

_irritability_ ruins our whole fabric; nor can the deficiency be

supplied by human art or skill: of consequence we must abandon this

part of the system entirely, and come to something more cognizable

by our senses. It is impossible, however, to pass over in silence

the amazing inattention of the author, in imagining that on such

unintelligible principles he could explain other phenomena. “In the

state of _temporary exhaustion_ (says he) the fibre loses its tone, and

fails for want of irritability. The application of a stimulus while it

is in this state will not make it contract. Provided the stimulus be

not very strong, it will produce no effect at all, but in a short time

the irritable principle will accumulate afresh in the fibre, and then

it will again contract. It is only by little and little that the fibre

recovers its irritability. This truth, I dare venture to say, is as

new as it is striking. It unfolds a vast number of phenomena hitherto

inexplicable.” Here we have nothing but the pompous declaration of a

fact already well known; viz. that not only a _fibre_, but the whole

body, may be in a state of temporary insensibility, and yet recover

either of itself or by the use of external means. How many people have

fallen into a _syncope_, and yet recovered! How many limbs have become

paralytic, and in time recovered their sense and motion! Yet this is

all that we are informed of with so much parade and assumption of

novelty. We know that when a person is in a faint he is insensible to

ordinary stimuli, though very strong ones will rouse him; but what can

we infer from this? Nothing; only we see it is so. Does it avail us any

thing to be told that during the time of fainting the _irritability_ is

exhausted, and “in a short time the irritable principle will accumulate

afresh;” in which case the patient will no doubt recover, unless he

happens to be dead, which is the true meaning of an _irreparable

exhaustion_ of the irritability.

In speaking of the principle of irritability he expresses himself in

the following manner. “I think that the oxygen is absorbed by the

blood, and that the venous blood is oxygenated in the lungs during

respiration. The most celebrated naturalists and chemists are of a

different opinion: they think that the oxygen does not combine with the

venous blood. According to them, this last loses carbon and hydrogen,

and recovers the bright colour natural to it, without absorbing any

thing from the atmosphere.... After having a long time attended the

phenomena of respiration, and made many experiments upon this subject,

I think it may be concluded that one part of the oxygen of the vital

air combines with the venous blood, of which it changes the black

colour, and makes it vermilion;[67] the second part of the oxygen

unites with the carbon contained in the carbonic-hydrogen gas, which

exhales from the venous blood, and forms carbonic acid air; a third

part unites with the carbon of the mucus, contained in great quantities

in the lungs, and which is continually decomposing; this part also

forms carbonic acid air; a fourth part of the oxygen combines with the

hydrogen of the blood to form water.”

[67] Here Dr. Beddoes, from whose publication this account of

Girtanner’s memoir is taken, has the following note: “Dr. Goodwyn

had proved this before. Could Dr. Girtanner be ignorant of his

experiments?” In justice to myself, however, I must observe that this

very doctrine had been published in the Encyclopædia Britannica long

before either Dr. Goodwyn or Dr. Girtanner had made any experiments

on the subject. It may still be seen under the article BLOOD, and

reasons are there given for supposing that only one part of the

oxygen, viz, the elastic part, can be absorbed.

On this theory I shall only observe, that though I lay claim to the

former part, I allow the Doctor all the latter part to himself;

particularly where he speaks of the _formation_ of water to be exhaled

during respiration. The air in question consists of two parts, like

_fixed air_ already mentioned. One of these is capable of being

attracted, condensed, or united with certain substances; the other

vanishes, leaving no other traces of its having ever existed, but heat,

greater or less according to circumstances. When the air is taken into

the blood, one part of it undoubtedly combines with something thrown

out by the lungs, and forms _fixed air_, of which our breath contains

a considerable quantity. We know certainly that the condensable part

of fixed air is formed out of the condensable part of the oxygen,

with certain additions. As therefore great part of this condensable

oxygen is thrown out in fixed air at every expiration, it is natural

to suppose that all of it is so: at least we cannot know the contrary

without a series of very difficult and tedious experiments, which have

never been made by Dr. Girtanner or any body else. But if the whole

of this condensable part be thrown out, none can enter the blood by

the breath; and consequently whatever true oxygen may afterwards be

expelled from that fluid, must be a factitious substance, formed either

during the artificial process, used for distilling it, or by a natural

process In the body itself. It is not therefore at all probable that

the oxygen which flesh emits in distillation can be derived from the

air by respiration.

Another and more probable source is the food and drink we take; all

of which are more or less impregnated with air of different kinds,

particularly fixed air. This, we know, very readily condenses, and

certainly will do so when taken into the body. In this state it not

only may, but certainly will, pass into the blood, and through all the

different parts of the body, until, having accomplished its purpose,

whatever that may be, it is thrown out by insensible perspiration, as

has been already explained.

The conclusions drawn by Dr. Girtanner from his experiments are,

1. That the change of colour which the blood undergoes during the

circulation is not owing to its combination with hydrogen air[68]. 2.

The deep colour of the blood in the veins is owing to the _carbon_ it

contains. 3. That the vermilion colour of the arterial blood proceeds

from the oxygen with which the blood is conjoined during its passage

through the lungs. 4. That respiration is a process exactly analogous

to the combustion and oxydation of metals; that these phenomena are

the same, and to be explained in the same manner. 5. That, during

circulation, the blood loses its oxygen, and charges itself with

_carbonic hydrogen_ air, by means of a double affinity. 6. That, during

the distribution of the oxygen through the system, the heat which was

united with this oxygen escapes; hence the animal heat. 7. That the

great capacity of arterial blood for heat is owing to the oxygen with

which it is united in the lungs.

[68] Here it is necessary to observe, for the sake of accuracy and

perspicuity, that, in the new chemistry, the terms of which are

now very generally adopted, the words _oxygen_ and _hydrogen_ when

mentioned by themselves are not understood to signify any kind of

air, but what I have called the condensable part of the air. If

the word _air_ is added, then the whole substance of the fluid is

understood. But though this is the strict orthodox language of the

new chemistry, it is impossible to say whether every one who adopts

the terms be sufficiently careful in this respect. Indeed this is

one out of many inconveniences that might be pointed out which

have arisen from this nomenclature; for thus the mere omission of

a monosyllable, which may happen in numberless instances, totally

perverts the meaning of the author, and may of course subject him

to unmerited censure. Besides, it is not to be known, unless the

author tells us so, that he designs to observe this strictness, and

of consequence we must in multitudes of cases be uncertain of the

meaning of what we read. Thus, in the present instance, when Dr.

Girtanner speaks of _oxygen_, we know not certainly whether he means

the air in substance, or only one of its component parts. Probably

he means the condensable or solid part. If he does so, there must be

a very material difference between his theory and that laid down in

the Encyclopædia, and which is supported throughout this treatise. In

the latter it is maintained that the condensable part is thrown out

by the breath, being previously converted into fixed air, while the

elastic part enters the vital fluid, communicating to it not only the

red colour, but heat, and the principles of life and sensation, as

will be more fully explained in the sequel.

On these propositions, which constitute in a great measure the

fundamental principles of the doctrine of _oxygenation_ of the human

body, we may remark,

1. Nobody can reasonably suppose that hydrogen air is the cause of the

dark colour of the blood in the veins, because there is no source from

which it can be derived; and, besides, it is certain that no kind of

air can exist in its elastic state in the blood, without destroying

the life of the animal. Some experiments proving this are given by Dr.

Girtanner himself. It is true that an aerial vapour, of the nature of

_fixed air_, exhales from the body by insensible perspiration; but

there can be no doubt that this receives its elasticity only at the

surface of the body, and is expelled the moment it is formed. It has

indeed been proved, by undeniable experiment, that no air of any kind

exists in the larger veins; because a portion of a vein, included

between two ligatures, being cut out, and put under the receiver of an

air-pump, does not swell in the least when the air is exhausted, which

yet must be the case, did the smallest quantity of elastic air exist in

it.[69]

[69] _Hydrogen_ air is the same with that by Dr. Priestley called

_inflammable_ air. He also discovered the true composition of it.

Having included a few grains of charcoal in the receiver of an

air-pump, and exhausted the air, he heated it in vacuo by means of

a large burning glass. The charcoal was entirely volatilized and

converted into this kind of air. He found, however, that without some

small portion of moisture this volatilization did not take place.

2. When the Doctor asserts that the dark colour of the venous blood is

owing to the carbon it contains, he is in the first place chargeable

with the error of former chemists, who supposed that every thing which

could be extracted from any substance by fire, existed previously in

it, in that very form in which it is extracted by the fire; and in

the second place he speaks entirely at random, without even a shadow

of proof. Nay, he himself tells us, that he has repeated two of Dr.

Priestley’s experiments, which in the clearest manner demonstrate,

that neither the addition nor the abstraction of carbon, or any thing

else, give this dark colour to the venous blood. “A small glass tube

(says he) filled with arterial blood, of a bright vermilion, was sealed

hermetically,[70] and exposed to the light. The blood changed its

colour by degrees, and in six days became black as venous blood. The

same experiment was repeated, with this difference only, that the tube

was exposed to heat, and not to the light. The blood became black in

a shorter time.” In these experiments it is plain, that if the blood

contained oxygen at first, it did so at the last; the same with regard

to carbon. How came it then to pass, that without either evaporation of

the former, or addition of the latter, the change should be produced?

If the oxygen imbibed by the blood in the lungs was sufficient to

produce the red colour, why did it not preserve it? The case here is

precisely similar to what happens with the calx of silver. When that

metal is dissolved in aqua fortis, and again reduced to a solid form,

it appears as a white powder, and will preserve its colour if carefully

kept from the light; but if a vial be filled with it, and exposed to

the sun, that side on which the light falls will in a short time become

black, and this though the vial has been ever so carefully sealed.[71]

Formerly, chemists had a method of accounting for this appearance, as

well as that of the venous blood, by what they called the _evolution of

phlogiston_: but now that the very existence of phlogiston is denied,

we are deprived of this resource. But, whatever words we may use, it

is plain that in neither case have we any ideas affixed to them which

can make the matter at all more intelligible than it was before. But

with regard to the blood, we are at a considerable loss to understand

what the natural colour of it is; and indeed the question can only be

determined by examining the blood of a fœtus which has never breathed.

If the arterial blood of such a fœtus be of a dark colour, resembling

that in the veins of a grown person, we must look upon this to be

_natural_ to it, and we may as well inquire why a rose is red, or an

iris blue, as why the blood is of a dark, and not of a bright red. But,

if we find this dark red change to a bright scarlet in the arteries,

as soon as the child has breathed, we have as much reason to conclude

that the air occasions this superior redness, as that an acid is the

cause of a red colour in the syrup of violets, or an alkali of a green

colour in the same. Experiments are yet wanting to determine this

matter. Mr. Hunter has observed that “in such fœtuses as convert animal

matter into nourishment, they most probably have it (the colour of the

blood) influenced by the air, such as the chick in the egg, although

not by means of the lungs of the chick, we find the blood, in the veins

of their temporary lungs, of a florid colour, while it is dark in the

arteries.”--The probability therefore is, that the blood is naturally

dark; by the elastic principle of the oxygen that it is rendered

brighter; and that, this elastic principle being expended in the course

of circulation, the fluid reassumes its original colour.

[70] A glass tube is sealed hermetically, by heating the open end or

ends, till they become soft, and then closing them with a pair of

pincers.

[71] Thus letters, or other characters, may be curiously marked upon

the calx within the vial, by cutting them out in paper, and then

pasting them on the side to be exposed to the light. We may have them

in this manner either dark upon a white ground, or white upon a dark

ground.

3. Though enough has already been said to evince that the superior

redness of the arterial blood is derived from oxygen gas, we shall

still quote two instances from Mr. Hunter’s Treatise on the Blood,

which set this forth in the clearest manner; and these instances are

the more remarkable, because they demonstrate the phenomena not of

the _dead_, but of the _living_ body. 1. A gentleman in an apoplexy,

who seemed to breathe with great difficulty, was bled in the temporal

artery. The blood flowed very slowly, and for a long time. It was as

dark as venous blood. He was relieved by the operation; but, on opening

the same orifice in two hours, the blood flowed of the usual florid

colour. 2. A lady in an apoplexy was treated in the same manner, and

Mr. Hunter observed, that when she breathed freely, the blood from the

temporal artery assumed a bright red colour; but when her breathing

was become difficult, or when she seemed scarce to breathe at all, it

resumed its dark colour, and this several times during the operation.

4. Respiration is not, as Dr. Girtanner says, a process similar to the

combustion and oxydation (the calcination) of metals. Some of these

by calcination, and _all_ of them in the opinion of Dr. Girtanner,

unite with the condensable part of the oxygen contained in the air,

while the elastic part is dissipated in flame or heat. The reverse of

this takes place in breathing; for here the elastic part of the oxygen

unites with the blood, and makes it warm, while the condensable part,

uniting with certain particles to be thrown off from the body, passes

away in fixed air. Thus the process of respiraton does not resemble

the calcination of a metal (at least according to our author’s opinion

of that operation) but rather the inflammation of some combustible

substance; for in both cases a certain quantity of carbon is found

to be united with the basis of oxygen in the atmosphere, and thrown

off from the place of combustion; and thus a quantity of fixed air is

produced from every burning substance. Just so is it with respiration.

If the condensable part of the oxygen combined with the blood, then no

fixed air could be produced; or if any part of the oxygenous base was

absorbed, it must certainly be known by a proportional deficiency in

the quantity of fixed air produced. But there are no experiments made

with accuracy sufficient to determine this point. It is true that many

very able physiologists, as Borelli, Jurin, &c. have been of opinion,

that part of the air is absorbed in respiration; but when we come to

particulars nothing can be determined. Dr. Hales by experiment found

the quantity absorbed to be a _sixty-eighth_ part of the whole quantity

inspired; but, on account of supposed errors, he states it only at _an

hundred and thirty-sixth_ part. Between these two the difference is

so enormous, that we know not how to draw any conclusion from them.

The French chemists are more decisive, and agree pretty well with one

another. Chaptal calculates it at _three hundred and fifty-three_, and

La Metherie at _three hundred and sixty_, cubic inches in an hour.

Allowing these experiments to be just, the next question is, what

part of the air is absorbed. Lavoisier says, that it is the oxygenous

base, or the same with that which is absorbed in the calcination of

mercury. But how comes he to know this? Surely not in the same way

that he determines the absorption of it by mercury. In the latter case

he takes a certain quantity of mercury, includes it in another known

quantity of oxygen air, and heats the metal by means of a burning-glass

or otherwise: the consequence is, that the air is absorbed, the mercury

loses its fluidity, and is increased in weight. The metal gains the

_whole_ weight of the air absorbed; and, by another process, _all_ the

air and _all_ the metal, or very nearly so, may be obtained in their

original form. This experiment is so decisive, that nothing can be

said against it with any shadow of reason; but who _hath_ made, or who

_can_ make, similar experiments with the blood of a living man? Such

experiments indeed might be made, if _insensible perspiration_ did not

stand in our way. Common atmospherical air is about _eight hundred_

times lighter than water. A cubic inch of distilled water, according to

Dr. Kirwan, weighs _two hundred and fifty-three grains and a quarter_.

Oxygen air is somewhat lighter than common air: we shall therefore

suppose that six hundred inches of it are equal to an inch of water.

If then the blood absorb three hundred and sixty inches of air in one

hour, it will in twenty-four hours have absorbed eight thousand six

hundred and forty inches, equal in weight to fourteen inches of water

and two fifths, which according to Dr. Kirwan’s estimate is between

seven and eight ounces. But the quantity of matter insensibly perspired

in that time is so much greater, that no calculation can be made. Here

is one mode of determining the quantity of oxygen inspired totally

impracticable in the human body, though quite easy and practicable in

the case of mercury. The other mode of determining it by the expulsion

of oxygen from the blood is equally impracticable. Dr. Girtanner indeed

has expelled oxygen from flesh; but we know not in what proportion,

nor can we determine whence it came. With regard to this last, indeed,

there are two sources allowed by Drs. Beddoes and Girtanner themselves;

viz. the absorption of oxygen by the lungs, and the quantity taken in

with the aliment. A third source was also manifest from Dr. Girtanner’s

experiments; viz. absorption from the atmosphere; for, by exposure to

the atmosphere, flesh, which had once parted with its oxygen, became

again impregnated with it. In this case therefore we must acknowledge

that the uncertainty of the absorption by the lungs must be extremely

great. A certain quantity of oxygen is undoubtedly thrown out in fixed

air. How are we to determine this quantity? Certainly not by the first

reverie that happens to occupy our imagination. It is a problem, the

solution of which must be attended with the utmost difficulty. We must

know, in the first place, how much oxygen was contained in the air

_inspired_. In the second place we must know the quantity of fixed

air _expired_. In the third place we must exactly know the proportion

of oxygen contained in the fixed air thrown out by the breath. In the

fourth place we must determine whether, by the conversion of oxygen

into fixed air, any change is made in its bulk. For, if this shall be

found to be the case, we should be led to suppose an absorption or

augmentation of air when no such thing took place. This point therefore

ought to be determined with the utmost accuracy. In the fifth place we

must exactly know how much _azote_, _septon_, _phlogisticated air_ is

contained in the atmosphere inspired, and likewise in that expired.

In the sixth place, we must be assured that there are no other fluids

in the atmosphere capable of being absorbed by the lungs, excepting

oxygen and azote. Whether there are any others or not, hath not been

determined. From an expression of Dr. Fordyce, he would seem to be

skeptical on the subject. “The atmosphere (says he) is found to consist

of various vapours, of which air, or, as it has been called, pure

air, or respirable air, (oxygen air) forms at present about a fourth.

Gas (probably fixed air) forms some part;[72] but the greatest part

consists of one or more vapours, which, _without any positive quality,

but from that indolence which makes mankind in their researches attempt

to find a resting place, have been considered by many chemists as one

individual species, under the names of phlogisticated air_,” &c. In

the seventh place we ought to know what quantity of _pure oxygen_,

unconverted into fixed air, or whether any such, is thrown out by the

breath. That a quantity of this kind of air is really thrown out,

is probable, because we can blow up a fire with our breath, and by

a blow-pipe excite a most intense heat, capable of melting the most

refractory metals, platina excepted. It is true that the eolipile, by

the mere conversion of water into steam, will blow up a fire also;

though, if the access of external air be denied, the blast of the

eolipile will put the fire out. Probably the breath would do the same;

but even this cannot be accounted a decisive proof of the oxygen being

totally exhausted; for the moist vapour with which the breath abounds

may extinguish the fire, even though some small quantity of oxygen

should remain in it. It is not, however, our business at present to

enter minutely into such discussions. From what has been already said,

it is evident, that the absorption of oxygen by the blood, instead of

being indubitably established, is of all things the most uncertain; the

requisites for determining it being absolutely beyond the investigation

of any person, however accurate. We may indeed, with great labour and

trouble, determine that some part of the air is absorbed in breathing;

but what that part is, we are unable to discover from any chemical

investigation. The opinion of the simplicity of metals, and their being

reduced to a calx by the adhesion of oxygen, has been so implicitly,

and in a manner universally, received, that it has given a new turn

to physiology, so that, by a kind of analogical reasoning, the human

body has been reduced to a mere chemical apparatus, the operations of

which may be calculated as we can do the event of experiments in a

laboratory. But, after a very long and tedious contest, Dr. Priestley

seems at last to have overthrown this doctrine of oxygenation, even in

the inanimate parts of the creation; so that we can much less apply it

to the doctrines of life and animation. His experiments are published

in the third number of the Medical Repository, volume II, and fully

demonstrate, that, though mercury absorbs oxygen during calcination,

this is not the case with all metals; that in many cases the oxygen

will unite with other substances in preference to the metal, which last

is nevertheless reduced to a calx as though it had united with the

oxygen; that in many cases the addition of weight gained by the calx

is owing to mere water, &c. He has likewise shown that phlogisticated

air (azote) is not a simple substance, as has been taught by the new

chemists, but consists, as well as fixed air, of an union of oxygen

with carbon, or at least with the black matter of burnt bones, with

which he made the experiment. These aerial fluids therefore being

so easily convertible into one another, and the uncertainty of the

changes in bulk which may occur in consequence of these conversions

so great, it is impossible to say whether a portion of the atmosphere

in substance, i. e. both oxygen and azote, is absorbed, as physicians

formerly supposed; or whether a portion of oxygen air alone be

absorbed, as Dr. Beddoes supposes; or whether only the elastic

principle itself is absorbed, and the diminution in bulk made in

consequence of the conversion of oxygen into fixed air; I say, these

matters depend on circumstances so much beyond the reach of our senses,

that if we come to any probable conclusion upon the subject, it must be

by analogical reasoning from other known facts, not from experiments

made directly upon the living body; which, in their own nature, must,

always be extremely vague and uncertain.

[72] It is now acknowledged that common atmospherical air contains

a portion of what Dr. Black and Dr. Priestley have called fixed

air; but this portion is so small (not more than one fiftieth part,

according to Dr. Anthony Fothergill’s Prize Dissertation, and _none

at all_, according to Dr. Beddoes) I say, this proportion is so

small, that we cannot suppose it to constitute the quantity of fixed

air thrown out by the breath, which is very considerable. Besides,

fixed air, of all others, is the most readily absorbed; and, indeed,

if we could admit of absorption of any basis of air in the present

case, it certainly ought to be that of fixed air; but where such a

quantity is thrown out, we cannot well admit of any absorption.

5. That, during the circulation, the blood charges itself with carbonic

hydrogen air, is an assertion which cannot be easily admitted. It has

already been observed, that, by the air-pump, venous blood does not

appear to contain any elastic fluid whatever; and it is also certain,

that animals cannot bear any quantity of air injected into their veins.

Dr. Girtanner himself tried several kinds, and all of them proved

fatal. Having injected a considerable quantity of oxygen air into

the jugular vein of a dog, the animal raised most terrible outcries,

breathed very quickly, and with the utmost difficulty; by little and

little his limbs became stiff, he fell asleep, and died in less than

three minutes. On injecting into the vein of another dog a small

quantity of phlogisticated air, the animal died in twenty seconds.

With carbonic acid gas (fixed air) a third dog died in a quarter of

an hour. A fourth was killed in six minutes by nitrous air.[73] From

these experiments, had no others ever been made on the subject, it

seems very probable, that no species of air can be safely admitted

into the blood in its elastic state. If any such therefore should

naturally be produced in the body, it must either be instantly thrown

out, or disease must ensue. Such objections to the Doctor’s theory

are so natural, that we might have thought he would have foreseen and

provided against them. Instead of this he grounds the whole upon such

slender evidence as could not be admitted in the most trifling matter.

“An incision (says he) was made in the jugular vein of a sheep, and

the blood which came from it was received into a bottle filled with

nitrous air. When the bottle was half filled, it was closed. The

blood coagulated immediately, and a separation of a great quantity

of blackish serum took place. The day after, on opening the bottle,

a very strong smell of nitrous ether (dulcified spirit of nitre) was

perceived, the nitrous air having been changed in part into nitrous

ether by the carbonic hydrogen gas of the blood. This experiment

proves, beyond a doubt, that the venous blood contains carbonic

hydrogen air; and that this air is not very intimately mixed with it,

but may be expelled with the greatest ease.”

[73] Nitrous air is that suffocating vapour which arises when aqua

fortis is poured upon metals. When taken into the lungs it destroys

animal life more quickly than any other species.

On reading the Doctor’s account of this experiment; it must be very

obvious, that, however decidedly he may be of opinion that it proves

_beyond a doubt_ the existence of hydrogen air in the venous blood,

yet there is not one solid reason; from what he says, for supposing

any such thing. How can any man determine from the mere _smell_ of

_sheep’s_ blood taken out of the body of the animal, and mixed with

a poisonous vapour, what is the composition of _human_ blood in the

living body? In the case of any substance suspected to contain elastic

air, the air-pump will always afford an _experimentum crucis_. But we

know that venous blood does not yield any elastic vapour by the pump:

if instead of blood, however; we should fill a portion of vein with

beer, cyder, or other fermented liquor, it would instantly discover,

by its swelling up, that it really contained air in an elastic state.

If then from the tumefaction of the vein when filled with fermented

liquor we conclude that the latter contains fixed air, why should we

not, from the non-tumefaction of it when filled with blood, conclude

that the vital fluid contains no air? If Dr. Girtanner was so well

assured that the venous blood contains hydrogen air, he ought to have

expelled some of it from a portion of the blood, noted the difference

between the blood which had lost its air, and that which had not, and

then, by adding the air to it again, restored the blood to its former

state. Nothing less then recomposition can prove the truth of a

chemical analysis; as division can only be proved by multiplication, or

multiplication by division.

From all that has been said, we may fairly conclude, that no proof can

be brought sufficient to prove the existence either of oxygen air or

any other species of aerial fluid, in its elastic state, in the blood.

Neither can we prove that any part of the condensable part of oxygen

air is received by the breath in the lungs. It is, however, probable

that this condensable part may be received into the stomach with our

food; that having passed through the various channels of circulation,

and arrived at last at the surface, it there resumes its aerial nature

by combining with the superfluous heat of the body, and is evaporated

through the pores of the skin by insensible perspiration. The aerial

vapour which passes off by these pores indeed has been discovered to

partake of the nature of _fixed air_; but we know that this species

of gas always contains the basis of oxygen, being indeed composed of

it; and whether the oxygen be taken into the body in its pure state

or not, the result would undoubtedly be the same; for an union would

be formed between it and the carbonic particles to be thrown off from

the body. But thus we can never suppose the basis of oxygen or any

other air to be a permanent part of the composition of our bodies; nor

can the quantity of it be augmented by breathing any kind of air. The

readiest way to increase the quantity seems to be by drinking fermented

liquors. Thus, if the body is too hot, the superfluous heat will have

a proper subject to act upon, viz. the condensable part of the fixed

air; and hence we may perhaps account for the very grateful and cooling

sensation produced by drinking these liquors in some diseases. With

respect to the existence of _carbon, charcoal_ or _hydrogen_ in the

blood, it is probable that it exists in equal quantity at all times,

being indeed the fundamental material of the whole body, and probably

only a modification of that _dust_ from whence man was originally

taken.[74] When the blood therefore grows very black, when the teeth

are covered with a black sordes, the hands become foul, &c. we may say,

indeed we too surely _feel_, that, in such cases, there is a propensity

in the body to return to its original state of dissolution; but there

is not one solid reason for supposing the proportion of its materials

to be varied; that there is a collection of _oxygen_ in one part,

_hydrogen_ in another, or in short that nature can admit of any such

disproportion taking place.

[74] In one of Dr. Priestley’s papers above quoted he says, that

_charcoal_ is entirely of _vegetable_ origin; but the conversion of

vegetable into animal matter which we daily see is an undoubted proof

that there cannot be any essential difference between them. Even the

bones are undoubtedly produced from vegetables in such animals as

feed upon vegetable substances; so that even the calcareous earth

they contain is plainly of vegetable origin. We may say indeed that

the calcareous particles had a previous existence in the vegetables

used by the animal as food; but we may say the same of the particles

of the blood, flesh, horns, &c. Besides, Dr. Priestley has shown that

_every_ particle of charcoal may be volatilized into inflammable

air, with as great accuracy as any human experiment can be made; so

that in this case the calcareous particles, if any such there were,

showed themselves to be as much charcoal as the rest. In the 74th

volume of the Philosophical Transactions, Mr. Watt has shown, that

dephlogisticated spirit of nitre may be changed into the smoking and

phlogisticated kind by means of red-lead or magnesia alba, as well

as by charcoal; of consequence there can be no essential difference

even there. In short, so wonderful and multifarious are the

transforming or metamorphosing powers of nature, that every attempt

to find out a substance upon which these powers cannot act, will be

found altogether vain, and our best conducted and most plausible

experiments, made with a view to discover the ultimate composition or

what we call the elements of bodies, will be found mere inaccuracy,

bungling and blunder.

6. We must now consider Dr. Girtanner’s account of the origin of

animal heat, which is, that, “during the distribution of the oxygen

through the system, the heat which was united with this oxygen

escapes; hence the animal heat;” and, “that the great _capacity_

of the arterial blood for heat is owing to the oxygen with which

it is united in the lungs.”--This leads us to consider in a more

particular manner the doctrine of heat, a subject hitherto much less

investigated than the importance of the subject requires. What little

we do know of this matter seems to be almost entirely owing to Dr.

Black, who hath discovered some very remarkable phenomena unknown to

former philosophers. His discovery here, as in that of fixed air, was

accidental. Making experiments on the water of different temperatures,

he found that the mixture would always be an arithmetical mean betwixt

the two quantities mixed. Thus, on mixing water at 50 degrees with

an equal quantity at 100, the temperature of the mixture would be 75

degrees; but if instead of using water only he took snow or ice for

one of the quantities, the mixture was no longer an arithmetical mean

betwixt the two temperatures, but greatly below it; so that a quantity

of heat seemed to be totally lost and in a manner annihilated. His

attention was engaged by this unexpected phenomenon, and, prosecuting

his experiments, he found that, when water was converted into ice, it

really became warmer than it was before; and, by keeping the fluid

perfectly still during the time that cold was applied, he was able

to cool it to 27 degrees of Fahrenheit’s thermometer, which is five

degrees below the freezing point; but on shaking this water so cooled,

it was instantly converted into ice, and the thermometer rose to 32. On

reversing the experiment he found that mere fluidity in water is not

sufficient to melt ice. A considerable degree of heat is necessary;

and even when this is previously given to the water, the whole becomes

as cold as ice by the time that the ice is melted. The result of his

experiments in short was this: Water, when frozen, absorbs an hundred

and thirty-five degrees of heat before its fluidity can be restored:

that is, supposing a pound of ice at the temperature of 32 to be mixed

with a pound of water at the temperature of 32, by adding 135 degrees,

so that the temperature of the water is augmented to 167, the ice will

indeed be melted, but the temperature of the whole quantity of liquid

will be reduced to 32. In this case therefore the heat manifestly

assumes two different modes of action: one in which it acts internally

upon the substance of the body, without being sensible to the touch,

while in its other state it hath no effect upon the internal parts, but

affects bodies on the outside. The former state therefore the Doctor

distinguished by the name of _latent_, the latter by that of _sensible_

heat.

The same theory was applied to explain the doctrine of evaporation, and

that in the most decisive and satisfactory manner. The Doctor found,

that, in the distillation of water, much more heat was communicated

to that in the worm-tub of the still, than could be supposed necessary

to raise the water distilled to 212 degrees, which is the utmost that

water can bear. In prosecuting the experiment he found the quantity of

heat absorbed by the water, when raised into vapour, truly surprising;

no less than _a thousand_ degrees; an heat more than sufficient to

have made the whole quantity of fluid that came over red hot. Some

objections, however, were made to this theory, even by the Doctor’s

friends. Mr. Watt, particularly, though he could not deny the theory

derived from Dr. Black’s experiments, yet suggested one, which, had

it proved successful, would have overthrown the whole. It was this:

Let water be distilled _in vacuo_, where it boils with a heat of 97

degrees, and the operation must be carried on with much less fuel, and

with much greater ease, than in the common mode. It was said that, in

this experiment, Dr. Black was equally concerned with Mr. Watt; but,

in a personal conversation with the Doctor himself, he assured me

that he had no farther concern than foretelling that the experiment

would not succeed, which it seems did not. The event was as follows:

Mr. Watt, determining at all events to try the experiment, caused to

be made a copper retort and receiver, joined together in one piece.

In the receiver he pierced a small hole, and, heating both retort and

receiver, plunged the latter into cold water. The consequence was,

that a considerable quantity of water entered the vessel, and was

easily poured back into the retort, as a subject for distillation. A

fire being now applied, the water was soon raised into steam, which

filled both retort and receiver, and in a great measure expelled the

external air. The small orifice in the receiver being now closed, and

the receiver itself plunged into cold water, the distillation went

on _in vacuo_; for, as soon as any of the steam was condensed, the

space which it had occupied (according to Dr. Black _one thousand and

sixty-six_ times more than the original water) was become absolutely

empty, and more steam, rarefied, not by any quantity of sensible heat,

but merely by that which it contained in a _latent_ state, would occupy

the place of the former. The event of the experiment showed the truth

of Dr. Black’s theory. The water boiled, and steam was raised as well

as if access had been given to the air; but with this difference,

that the upper part of the distilling vessel was never heated above

what the hand could easily bear. With the water in the cooler it was

quite otherwise. It became hot as usual, and, by the quantity of heat

it received, plainly demonstrated that the vapour, though destitute

of most of its _sensible_ heat, yet contained an immense quantity in

a _latent_ state. The saving of fuel therefore in the practice of

distillation, which was Mr. Watt’s object in making the experiment, was

quite trifling, and not equal to the trouble of filling the retort with

liquid.

The doctrine of latent heat thus established, furnished a solution of

many phenomena which could not formerly be explained in a satisfactory

manner. Thus the melting of all kinds of substances was found to be

owing to an absorption of heat, while their condensation was attended

with the contrary. Fluidity in all cases was explained on the same

principle; and the more heat that was absorbed, the more fluid the

matter became. Thus water, when in a condensed or solid state, absorbs

135 degrees of heat before it becomes fluid. A thousand degrees more

convert it into vapour, and at last, by passing through the intense

heat of a glass-house furnace, it is converted into a brilliant

flame, and augments the heat of the furnace to a great degree. Hence

the practice in glass-houses of throwing water into the ash-hole,

the vapour of which, by passing through the burning fuel, makes the

furnace much hotter than it was. In a similar manner were explained

the phenomena of crystallization, the ductility of metals, the heat

produced by hammering them, and the hardness produced by the operation,

as well as the operation of annealing, &c. One other phenomenon, a very

curious one, shall be noticed, on account of its being connected with

the subject of this treatise. It is this: Let a small vessel filled

with vitriolic ether be put into a larger one of water, and both

included in the receiver of an air-pump. On exhausting the air, the

ether boils, and is converted into vapour, while the water freezes.

This shows that heat does not always act equally upon surrounding

bodies, but has a tendency to enter some in preference to others;

and from other experiments it appears, that this property has a

considerable connexion with the density of the bodies concerned.

Thus one step was gained, and it was universally admitted that heat,

in some cases, entered bodies, and in others was thrown out of them;

but now the question arose, What is heat; and by what laws is it

regulated, or from what source is it derived? Here Dr. Black himself

was at a loss; for, as he supposed _cold_ to be a mere non-entity,

and only to consist in a comparatively smaller degree of heat, some

phenomena occurred which would not easily admit of solution upon such

an hypothesis. With these Dr. Black did not meddle much, but others

were bolder. Dr. Irving, Professor of Chemistry at Glasgow, undertook

to explain the whole mystery of latent heat upon the single principle

of attraction. One of the most puzzling phenomena in the way of Dr.

Black’s theory had been, that in some cases heat and cold seemed to

repel each other, and a very remarkable instance of this was, that,

in the morning, a little before sunrise, when the rays of light pass

through the atmosphere, a little above the surface of the earth, the

air then becomes manifestly colder than even at midnight. Dr. Irving’s

explanation of this was, that the sun’s rays _attracted heat from

the atmosphere_, and thus rendered it colder. Such at least was the

explanation given in an inaugural dissertation by Dr. Cleghorn, one of

Dr. Irving’s scholars; for the Doctor himself delivered his opinions

only to them. In other cases he supposed that different substances

had different _capacities_ for receiving heat; and, of consequence,

should the _form_, or rather the _internal constitution_, of the body

be changed, the _capacity_ of it for receiving heat must also be

changed; and as an attraction subsists, or is supposed to subsist,

between heat and all other substances, it is plain that while this

attraction subsists, if the capacity of any substance for receiving

heat be augmented, it will imbibe much more than it would have done

had its former constitution remained. Thus _water_ in its liquid state

contains a certain quantity of heat; we may therefore say that water

has a capacity for receiving heat equal to one to ten, or what we

please. Vapour has a capacity for containing heat ten times greater

than water. Water therefore, when converted into vapour, will imbibe

ten times the quantity of heat that the water contains; and, again,

on being re-converted into water, the _capacity_ becoming what it was

before, the superfluous quantity must be thrown out, as in Dr. Black’s

experiments. In like manner, when a metal is melted by the fire, the

capacity of it for receiving heat is changed: of consequence a great

quantity is imbibed, and again expelled by the change of _capacity_

which takes place on its becoming solid; and thus, from the change of

capacity, in different substances, every phenomenon was solved.

This doctrine of _capacities_ did not give general satisfaction.

Dr. Black himself said of it, that it was neither _probable_ nor

_ingenious_;[75] notwithstanding which, it continued to be received,

and even very generally adopted. Dr. Crawford, so well known for his

writings on this subject, has adopted the idea, and Dr. Girtanner, in

the passage above quoted from him, appears to be of the same opinion.

The doctrine, however, had several opponents, among whom were the

Monthly Reviewers. In their account of Nicholson’s First Principles

of Chemistry, they express themselves in the following manner: “We

only wish, that, in the doctrine of heat, he had avoided, which he

might easily have done, Dr. Crawford’s idea of bodies having different

_capacities_ for heat. In the melting of ice, for instance, a quantity

of heat is absorbed, without any increase of the temperature, that

is, without making the water sensibly warmer than the ice before its

liquefaction; which is said to be owing to the water having a greater

_capacity_ for heat, or being able to _hold more_ of it, than the

ice; and, in like manner, when converted into vapour, its capacity

is further increased, or it can hold more still. This appears to us

a very unchemical, and a very inadequate idea of the matter: for,

admitting water to have a greater capacity than ice, how is the change

from one state to the other to be effected? Can the properties which a

body is found to possess, after a change has taken place, be assigned

as a cause of the change itself? Or will it be said, that the heat

first enlarges the capacity, and then hides itself in that capacity

so enlarged? We should think it much better to say, consonantly with

the phenomena of other combinations in chemistry, that a certain

quantity of heat, uniting with the ice, first _liquefies_ it, as a

certain quantity of acid only neutralizes an alkali; that if any

surplus quantity must be introduced, that surplus, remaining free and

uncombined, must act and be sensible as heat in the one case, and

acid other; and that different bodies require different quantities of

heat or acid to be combined with them, for producing the changes in

question.”[76]

[75] These words are to be found in the M. S. Copies of his lectures

circulated at Edinburgh. Dr. Black himself never published any thing

to the world upon the subject.

[76] Monthly Review, for 1790, p. 165.

Thus the Reviewers, as well as others, reasoned _a priori_, and several

facts were adduced to prove that no such changes in capacity could take

place. But however strong the arguments adduced, or however plain the

experiments might be, little or no notice was taken of them, and the

enlargement or diminution of _capacities_ has been repeated, seemingly

by rote, from one author to another, without the least inquiry or

investigation. Dr. Girtanner indeed says that “the oxygen united with

the arterial blood in the lungs” is the cause of the _great capacity_

of the arterial blood for heat. But this is assigning a very doubtful

cause for a very doubtful effect. He ought to have proved in the

first place that arterial blood really has this capacity; for its

being _hotter_ than the blood of the veins, only shews that it parts

with more heat to surrounding bodies than venous blood does; which

is a proof that it contains _less_ heat, if there be any difference,

than that of the veins. But the truth is, that the _capacity_ for

containing heat depends neither on the oxygenation nor hydrogenation

of a fluid, but upon its density. The more fluid and the more easily

expansible into vapour that any substance is, the greater quantity

of heat it is capable of containing, and _vice versa_. This has been

fully ascertained by Mr. William Jones, an English clergyman, whose

observations on the generally received system of philosophy contain

many particulars worthy of attention. From his experiments it appears

that a piece of red-hot iron, thrown into water, imparts much less

_sensible_ heat to it, and is itself much more effectually quenched,

than by throwing it into an equal quantity of quick-silver of the same

temperature with the water. As the quick-silver therefore becomes much

hotter to the touch than water does upon throwing a piece of red-hot

iron into it, and as the iron itself is much more imperfectly quenched

by the metal than by the water, it follows that the latter is capable

of containing much more heat than the former. But such experiments

are not applicable to the blood. Though that of the arteries may be

somewhat hotter than the venous blood, yet the reason is obvious. The

heat is communicated directly to the arterial blood in the lungs; but

during the circulation a part of it evaporates, and the farther distant

any part is from the lungs, the more cool will the vital fluid be,

without regard to any alteration of _capacity_, which indeed never can

be shown to exist.

But the most decisive experiments against any supposed alteration in

the capacities of bodies for containing heat are those lately tried

by Count Rumford, and related in the Philosophical Transactions for

1798. His attention to this subject was engaged by observing the great

degree of heat acquired by a brass gun during the time of boring

it,[77] and still more by the intense heat (much greater than that of

boiling water) of the metallic chips separated from it by the borer.

From a consideration of these things he was naturally led to the

following inquiries. “Whence comes the heat actually produced in this

mechanical operation? Is it furnished by the metallic chips which are

separated by the borer from the solid mass of metal? If this were the

case, then, according to the modern doctrine of caloric, the _capacity

for heat_ of the parts of the metal so reduced to chips, ought not

only to be changed, but the change undergone by them be sufficiently

great to account for _all_ the heat produced. But no such change had

taken place; for I found, that by taking equal quantities by weight of

these chips, and of thin slips of the same block of metal, separated

by means of a fine saw, and putting them at the same temperature, that

of boiling water, and putting them into equal quantities of cold water

(that is to say, at 59-1/2 of Fahrenheit) the portion of water into

which the chips were put, was not, to all appearance, heated either

less or more than the other portion in which the chips were put.”

[77] Count Rumford was superintendant of boring the cannon in the

workshops of the military hospital at Munich.

From this experiment, several times repeated with the same result,

Count Rumford inferred, that the heat could not possibly have been

furnished at the expense of the latent heat of the metallic chips. He

then proceeded to ascertain “how much heat was actually generated by

friction, when a blunt steel borer being so forcibly shoved (by means

of a strong screw) against the bottom of the bore of the cylinder,

[of the machine in use] that the pressure against it was equal to

the weight of about ten thousand lb. avoirdupois, the cylinder being

turned round on its axis (by the power of horses) at the rate of about

thirty-two times in a minute.” In this experiment the metallic dust

or scaly matter detached from the cylinder by the borer weighed only

837 grains troy; but, says the author, “Is it possible that the very

considerable quantity of heat produced in this experiment (a quantity

which actually raised the temperature of above 113 lb. of gun-metal

at least 70 degrees of Fahrenheit’s thermometer, and which of course

would have been capable of melting 6-1/2 lbs. of ice, or making near

five pounds of ice-cold water to boil) could have been furnished by

so inconsiderable a quantity of metallic dust, and this merely in

consequence of a _change_ of its capacity for heat? As the weight of

this metallic dust (837 grains troy) amounted to no more than one

948th part of that of the cylinder, it must have lost no less than

948 degrees of heat to have been able to raise the temperature of the

cylinder one degree; and consequently it must have given off more than

_sixty-six thousand, three hundred and sixty_ degrees of heat to have

produced the effects which were actually found to have been produced in

this experiment.”

It was next considered whether the air did not contribute to the

generation of this heat; and our author determined that this could not

be the case; because the quantity of heat generated was not sensibly

diminished when the free access of air was prevented. From another

experiment it appeared that the generation of the heat was neither

prevented nor retarded by keeping the apparatus immersed in water. Here

the friction generated so much heat, that in one hour the temperature

of the water surrounding the cylinder was raised from 60 to 107 degrees

of Fahrenheit. In half an hour more it was raised to 142; at the end

of two hours to 178; at two hours 20 minutes to 200; and in two hours

and a half it boiled.[78] On the whole, Count Rumford concludes, that

“the quantity of heat, produced equably by the friction of the blunt

borer against the bottom of the hollow metallic cylinder, was _greater_

than that produced equably in the combustion of _nine wax candles_,

each three quarters of an inch diameter, all burning at the same time

with a clear, bright flame.” From all these experiments, however, our

author does not draw any certain conclusion. “What is heat? (says he.)

Is there any such thing as an _igneous fluid_? Is there any thing that

can with propriety be called _caloric_? The heat produced, in the

author’s experiments, by the friction of two metallic surfaces, was

not furnished by small particles of metal, detached from the larger

solid on their being rubbed together. It was not supplied by the

air, because the machinery in three experiments was kept under water,

and the access of atmospherical air completely prevented. It was not

furnished by the water which surrounded the machinery, because this

water was continually receiving heat from the machinery and could not

at the same time be giving to and receiving heat from the same body;

and because there was no chemical decomposition of any part of this

water.” At last he observes, that the source of this heat, whatever it

is, must evidently be inexhaustible, adding, that “any thing, which

any _insulated_ body, or system of bodies, can continue to furnish

_without limitation_, cannot possibly be a _material substance_; and

it appears to me to be extremely difficult, if not quite impossible,

to form any distinct idea of any thing capable of being excited and

communicated, in the manner the heat was excited and communicated in

these experiments, except it be MOTION.”

[78] The quantity was two gallons and a quart, wine measure.

On this last paragraph, however, it is obvious to remark, that the

whole force of the argument rests upon an _insinuation_, that the

cylinder and borer were _insulated_, or cut off from all communication

with any other material substance. Had this been the case, then no

doubt it would follow that an _endless_ supply of any thing _material_

could not be furnished by them; but if, as Dr. Boerhaave and many other

learned and intelligent persons have supposed, fire be an element

universally present, and which becomes sensible to the touch only in

consequence of a particular mode of action, it will follow, that no

substance in nature can be _insulated_ with respect to it; but, in

whatever place, and for whatever length of time, any substance shall

be affected in such a manner as to agitate this fluid, there we shall

perceive a production of heat _without limitation_, even though heat

itself be no more than the action of a fluid essentially _material_,

though invisible to us.

Considerations of this kind occurred long ago to the writer of this

treatise, when by the nature of his employment it was necessary for

him to speculate upon these subjects. It could not then but appear

to him that the theory of Dr. Black was far superior to any that had

been published. The opinion of those who supposed fire to consist

in the vibratory motion of the particles of solid bodies, seemed

altogether untenable. It is impossible to explain the phenomena of

heat upon ordinary mechanical principles, because, with respect to

all terrestrial substances, heat constantly appears as an _agent_,

while they are merely _passive_; and no man can explain the nature of

a cause from its effect. Thus one of the most obvious effects of heat

is _expansion_, or enlargement of bulk, in such bodies as are heated.

But if from this fact we infer that the parts of elementary fire are

repulsive of one another, our reasoning is certainly erroneous. In like

manner, when we are not sensible of heat, we are not authorised to

conclude that it is not present; for Dr. Black has demonstrated that it

may be present in very great quantity, though indiscoverable either by

our senses or by a thermometer.

But, with regard to the theory published by Dr. Black himself, it

is evident that, though one part of it rests on the solid basis of

experiment, the other is founded entirely upon hypothesis, and that too

an hypothesis which cannot admit of being proved by any experiment,

viz. that _cold_ is a mere negative, and hath no real existence in

nature. Among many phenomena which militate against this opinion, the

following experiment of M. Geoffrey seems to be the most remarkable. He

took a small bason filled with water, and set it on a support in the

middle of a large tub of water, in such a manner that the temperature

of the water in the tub might communicate itself to that in the bason.

This being ascertained by a thermometer placed in the bason, he threw

a quantity of burning coals into the tub. The effect of this, on the

supposition that cold is a mere privation of heat, ought to have been,

that the heat of the coals, communicated to the water in the large

tub, would in a short time pervade the small bason, and affect the

thermometer there. The latter would therefore rise; but instead of this

it fell several degrees before it began to rise; for which it doth

not appear that any other reason can be assigned than that the cold

is partly repelled by the heat of the coals, and therefore, entering

into the small bason of water, it causes the thermometer to sink

previous to its rising. To the same purpose we may urge the phenomenon

already taken notice of, viz. that the sun’s rays, when passing at some

distance above the surface of the earth, cool the lower part of the

atmosphere. The natural solution is, that the heat of the sun partly

repels the cold downwards; and as for the doctrine of _attracting_ heat

from the atmosphere, Count Rumford has shown that this does not happen

in a case where we might with much more probability expect it; not

to mention the violence done to the common perceptions of mankind by

supposing the sun’s rays, which are most evidently the source of heat,

to have any occasion to _attract_ heat from the atmosphere or any thing

else.

Lastly, with regard to the _capacities_ of bodies for containing heat,

the doctrine appears to involve a radical error, of such enormous

magnitude, that it is impossible to make any thing of it. This is no

less than confounding the heat which flows out from bodies with that

which they contain as an essential part of their composition, and which

they cannot emit without being changed into some other form. Thus the

capacity of aqueous vapour for containing heat, according to Dr. Black,

is 1000 degrees; yet without decomposing the vapour it would have been

impossible to have known this; for vapour is often extremely cold to

the touch, and a thermometer immersed in it will sink greatly. In

short, all that we can know about the capacity of bodies for retaining

heat is, that they either continue to absorb it, or we may continue

to force it into them, till they be reduced to vapour. It is doubtful

whether they can receive more; for from the experiment with Papin’s

digester, formerly mentioned, it appears that the additional quantity

of heat, which the water was made to receive, very quickly left it as

soon as the steam had room to expand.

But, to come to a conclusion upon this subject: If we will investigate

the nature of heat, we must do it as in other cases, viz. by making the

_igneous fluid_, _caloric_, or what we please to call it, the object of

our senses; for we cannot reason fairly, or indeed come to any rational

conclusion at all, by doing otherwise. In this investigation it is

necessary to attend to the particulars mentioned by Count Rumford. The

fluid must be omnipresent in its nature, infinite in its quantity, and

equable, uniform and incessant in its action; as far as these epithets

can be applied to any material being. There are only two fluids which

we know that can answer to these characteristics. The one is the light

of the sun, which pervades all the celestial spaces; the other the

electric fluid, which penetrates every terrestrial substance. Both of

these produce heat, unlimited in quantity, as well as in duration,

provided their action be continually kept up. The mode in which both

produce heat is exactly the same, viz. by converging into a focus; and

the greater the quantity, the greater is the heat, and that without any

limitation either as to intensity or duration. With regard to the solar

rays, it has long been known that by concentration they would produce

heat; nevertheless it was unaccountably doubted whether the rays

themselves were the matter of heat. One objection to this was, that on

the tops of high mountains the air is exceedingly cold, though the sun

shines very bright. But this objection was founded upon an erroneous

notion that, wherever the matter of heat exists, there we must feel it;

which doth not follow any more than that wherever air exists there we

must feel a wind blowing upon us. _Wind_ is air in motion, and _heat_

is a more subtile fluid in motion. One demonstration of this is, that,

on the tops of the highest mountains, a burning lens or mirror will

set fire to combustible bodies as readily as in the vallies at the

foot of them. Neither has heat, properly so called, anything to do

with air. The focus of a burning-glass will heat bodies _in vacuo_ as

well as in the open air; and Sir Isaac Newton has observed, that if a

thermometer be included in the vacuum of an air-pump, it will acquire

the temperature of the room nearly in the same time that another will

when included in a similar glass without any exhaustion.

The science of electricity is but of late date; and most violent and

hypothetical disputes have taken place concerning the nature of the

fluid. Its luminous and burning properties naturally led a number of

people to suppose that it was elementary fire; but this was opposed by

others with as much violence as if there had been something criminal

in the supposition. The opposition, however, was founded upon the same

error with that about the solar light. It was imagined that wherever

elementary fire existed, there heat must be felt; and it was especially

urged, that electricity, though it produced light, did not produce

any heat, except when it exploded with such violence as to penetrate

the internal substance of bodies, agitating their particles, and by

this agitation producing heat. It has now, however, been found, that

the electric _aura_, as it is called, when made to converge in great

quantity to the point of a needle, will heat it to such a degree as

to set fire to gun-powder. This shows that heat is occasioned by the

convergence of this fluid to a focus, and to its divergence from it. In

the focal point, heat will always take place. From the experiments of

Hauksbee, Beccaria and Priestley, it likewise appears, that electricity

will render transparent the most opaque bodies, such as sealing-wax,

pitch, &c. which even the most intense light of the sun cannot do. As

to the intensity of the heat produced by it, experiments have shown,

that it cannot be exceeded even by that of the most powerful mirror.

Globules of gold have been vitrified, platina melted, and the most

infusible substances reduced to glass, by means of the electric shock.

From so many evidences, therefore, it appears to me impossible to

conclude otherwise than that the light of the sun and the electric

fluid are the same thing; and, according to the different modes in

which they act, they produce the phenomena of heat and light in all

their varieties, besides a multitude of other effects of which we

cannot have any perception. We may indeed, if we please, suppose that

some other thing exists which is heat itself, and that the light or

electric fluid sets in motion, attracts, repels, or acts otherwise upon

this unknown something; just as it comes into our heads to fabricate

our system. But, until our senses can discover in some way or other

this hidden substance, _reason_ will always suggest that it has no

existence. We may say that without such a supposition we cannot solve

the phenomena of heat. But do we ever expect to solve these phenomena;

or do we know all that the solar light and electric fluid can perform?

If we do not know what they _can_ do, neither do we know what they

_cannot_; and the invention of other fluids must be accounted not only

chimerical but useless.

A Treatise on the Plague and Yellow Fever With an Appendix, containing histories of the plague at Athens in the time of the Peloponnesian War; at Constantinople in the time of Justinian; at London in 1665; at Marseilles in 1720 · 第 4 / 13 章

5. In consequence of the chemical action of the medicine, the mode