But, to be more particular: on the subject of heat people have

embarrassed themselves more with philosophical reveries than by any

real difficulty, and rendered the matter more obscure than nature

has made it. We have already observed, that by the convergence of

light, or of electricity, heat is always produced. Here we can see the

mode in which the fluid acts, viz. first by _converging_, and then

_diverging_. When the light falls upon a solid body, it is evident,

that if it be allowed to flow out as easily as it flows in, no internal

agitation of the parts, or of any fluid contained in them, can take

place. Transparent bodies therefore are never heated. Again, if the

light be not allowed to enter the substance of a body, but is entirely

reflected, the body cannot be heated; and hence it is very difficult

to melt a polished metal even by a strong burning-glass. M. Macquer’s

burning mirror, which vitrified flints, could not melt silver. But,

when the light falls upon a body capable of allowing it to enter its

substance, at the same time that it cannot get out without difficulty,

it is plain that the force of the fluid will be exerted in order to

overcome that difficulty; the body will be expanded in all directions;

the fluid will be thrown out in the same manner, and the more that

the internal action of the light prevails over that power by which

the parts of the body cohere, the more will the phenomena of heat be

perceptible.

Again, let us suppose that the etherial fluid enters the substance of

any body capable of being dilated to a great degree, it is equally

plain that the action of the fluid must for some time be directed only

upon the internal parts, and consequently will be imperceptible on the

outside. This then is called _latent_ heat; and where the pressure on

the outside balances that on the inside no heat will be perceptible

to the touch. But by whatever means this balance is broken, heat will

instantly be perceptible; and experiments show that the balance may

be broken either by an increase of cold or heat. Thus, in the case

of water, the internal pressure remains equal to the external, until

the fluid is cooled to a few degrees below 32. The balance is then

broken, and the internal action prevails; a quantity of what is called

_sensible_ heat escapes, and the water is converted into ice. Again, at

the temperature of 32, little or none of the water evaporates; but by

the addition of heat, by which the internal action of the subtile fluid

we speak of becomes greater than the external, the water is converted

into vapour; and it is remarkable that the same effect takes place on

greatly augmenting the degree of cold; for the evaporation from ice,

even in frosty weather, is found to be very considerable.

On the whole, from innumerable experiments it appears, that there

exists in nature a certain invisible fluid, by the action of which,

when diverging from a centre, heat is produced in the central point.

By a certain other power this diverging force is limited, so that in

some cases it is not perceptible beyond the surface of the body in

which it acts, and then it is called _latent_ heat. In other cases

it is perceptible in a certain degree, and the degree in which it is

perceptible hath been called the _temperature_ or _sensible heat_

of the body. On mixing different substances together it is found,

that very often the proportions between the external and internal

actions are varied. This has been already observed, when giving an

account of Dr. Black’s discovery of _latent_ heat, viz. that when

snow and warm water are mixed together the temperature of the mixture

differs very considerably from the arithmetical mean between the

temperatures of the two substances employed. Dr. Crawford prosecuted

the experiment further, and found that there were few substances

which, on being mixed, did not shew a temperature different from

that of the arithmetical mean between the temperatures of the two

originally employed. This difference he unfortunately used as the

foundation of a rule for determining the _capacities_ of different

substances for containing heat, and upon this erroneous principle has

raised a superstructure, which upon no occasion can be of service to

science, but must always produce obscurity and confusion wherever it is

introduced.

With regard to the power which sets bounds to the expansion of the

fluid acting as heat, it is natural to think that it can be no

other than the same fluid acting in a contrary direction, or from a

circumference towards a centre; and thus we shall always find that the

same fluid, by limiting its own operations, may produce those phenomena

which have been hitherto deemed so difficult of explanation. In what

manner this limitation is in all cases effected, or indeed in any

case, we cannot pretend to explain. It is sufficient to observe, that

wherever there is a perpetual _efflux_ of any thing, there must be

also a perpetual _influx_ at the same time, and in proportion to the

one the other will be. These two are directly contrary to one another,

and, as we suppose the fluid to be universal, it is evident, that if

any part of it be put in motion in a particular direction, the rest

will press towards that part where the motion is, in order to keep up

the equilibrium. Hence we may easily account for the heat produced by

percussion or by friction. By hammering a piece of iron, as Dr. Black

justly observes, the fluid is forced out from between the parts of the

metal. The emission of this fluid in all directions is heat itself;

and no sooner is one quantity thrown out than another supplies its

place with great rapidity, and so on, until the pressure of the rest in

some way or other counteracts the emission of any more, and the heat

ceases. Just so with friction. The heat produced by it is always in

proportion to the pressure employed. By this pressure the parts of the

two substances are forced into such close contact, that an agitation

and emission of the fluid pervading their substance takes place. This

agitation, as we have already noticed, is heat itself, and, as long as

the friction is continued, more and more heat will be produced, without

any limitation, as Count Rumford has observed.

Some bodies have a greater disposition than others to emit this subtile

fluid; and these we say are naturally of a warmer temperature than

others. The _temperature_ is nothing else than the efflux of the fluid

from them, continually kept up by the action of the surrounding fluid.

By mixture with different substances the temperatures of various bodies

may be changed; by some the influx, and by others the efflux, may be

augmented. In the former case we say the body becomes colder, in the

latter hotter, than before; and in not a few cases the agitation of

the fluid becomes so great that the matter actually takes fire. In all

these cases, however, we can discover nothing more than the bare fact,

that so and so is the case. We know that the bodies do grow hot by the

convergence of the etherial fluid towards them, and its emission from

them; but why it should converge or diverge we know not.

Thus much with regard to heat in general. We must next consider another

fluid which has very generally been accounted the source and fountain

of heat, viz. _air_. This is indeed so much the source of heat in all

our operations, that it was natural to think it the only one; but

experiments have now determined that air itself is a mere creature of

heat and light;[79] for, by employing these in a proper manner, airs

or gases of all kinds have been produced. Thus, by exposing water in a

glass vessel for some time to the rays of the sun, a quantity of very

pure oxygen air may be obtained; by concentrating the sun’s rays upon

charcoal, inflammable air may be had; and by distilling, with a strong

heat, substances of various kinds, we may obtain a great variety of

aerial vapours. From all this we may reasonably conclude that heat,

attached to some other substance, dissolved in it in such a manner as

to become invisible, forms the substance of air. Heat therefore being

the agent in the composition of air, it is reasonable to suppose that

it is the agent in its decomposition also, or in its transformation

from one species to another, of which the conversion of oxygen into

fixed air by combustion is an instance. When air is taken into the

lungs the blood is warmed by the action of that invisible fluid,

which has already given elasticity to the air. In consequence of a

considerable quantity of this fluid being then converted from a latent

into a sensible state, part of the elastic principle must be lost, and

the air diminished in bulk. The reason why this must constantly take

place is, that part of the heat evaporates from the surface of the

body, during the course of circulation. Were it not so, the quantity

thrown out by the lungs would be exactly equal to that which the blood

received, and consequently there could be no diminution between the

bulk of the air expired and that which was inspired; but, on account

of the waste just mentioned, the blood must always receive somewhat

more than it gives out by the breath. Thus, while the air we breathe

continues the same, and the organization of the body is not changed,

the natural operations will go on smoothly, and health will continue;

but, as we have formerly observed, by an alteration of either of these,

disease must ensue; and we must now endeavour, from the principles laid

down, to examine the mode in which epidemic diseases, and particularly

the plague, may be produced.

[79] Dr. Priestley thinks water is an _essential_ in the composition

of air.

The air is so evidently connected with human life, that it has been

from the earliest ages accounted the source of pestilential diseases,

though, as none of the more obvious qualities of it, such as heat,

cold, moisture, or dryness, appeared to be connected with them, they

were generally supposed to proceed from the action of some unknown

natural cause, or from that of the Deity himself. Some, however, have

also been of opinion that plagues might originate from the obvious

qualities of the air in conjunction with certain effluvia from putrid

vegetable or animal bodies. Thus, in several plagues mentioned in

ancient history, we find swarms of dead locusts, grasshoppers, the

carcases of those slain in battle, crowded houses, and filth of all

kinds, assigned as causes. This opinion was adopted by Dr. Mead,

and he gives the following account of the origin of the plague in

Egypt. “Grand Cairo is crowded with inhabitants, who for the most

part live very poorly and nastily; the streets are narrow and close;

it is situate in a sandy plain, at the foot of a mountain, which, by

keeping off the winds that would refresh the air, makes the heats very

stifling. Through the midst of the city passes a great canal, which is

filled with water at the overflowing of the Nile; and, after the river

decreases, is gradually dried up: into this canal the people throw all

manner of carrion, filth, &c. so that the stench which arises from

this and the mud together is insufferably offensive. In this posture

of things, the plague every year preys upon the inhabitants, and is

only stopped when the Nile, by overflowing, washes away this load of

filth; the cold winds, which set in at the same time, lending their

assistance by purifying the air.” He then proceeds to account for the

plagues in Ethiopia in the manner above related, viz. by the prodigious

swarms of locusts, which sometimes occasion a famine by devouring the

fruits of the earth, and, when they happen to be cast by the winds

into the sea, occasion a pestilence; the putrefaction being heightened

by the intemperance of the climate, which here is so great that it is

infested with violent rains for three or four months together; and

it is particularly observed of this country, that the plague usually

invades it whenever rains fall during the sultry heats of July and

August. He next takes notice of what the Arabians say of the origin

of the plague in Ethiopia, viz. that it is brought on by unseasonable

moistures, heats, and want of winds. But, whatever truth may be in the

account given of the Ethiopic plagues, the testimonies already produced

in this treatise are sufficient to render it very doubtful, at least,

whether the plague ever does originate in Cairo, or any other place

in Egypt. Besides, if we once admit the existence of any thing as a

cause adequate to the production of a certain effect, wherever that

cause exists the effect ought certainly to follow, unless where we

plainly perceive something which prevents its action. It is not fair

reasoning to say that the action of the cause is prevented by something

unknown, for we might as well say that this unknown something is the

cause originally, and acts only upon certain occasions, of when it

thinks proper. Now, if the filth of the canal of Cairo be the cause of

the plague in that city, it ought to recur annually at the season when

that filth exists in greatest quantity, and in the most putrid slate.

Nevertheless we have the express testimony of Mr. Eyles Irwin, that at

the time he was in Cairo there had not been any plague for seven years.

The account he gives of it is a kind of contrast to that above quoted

from Dr. Mead. “_Misir al Kaira_, says he, or the _City of Anguish_, so

called from the frequent visits which it has received from the plague,

but commonly called _Grand Cairo_ by us, is situated in lat. 30 degrees

3 minutes N. on an artificial branch of the Nile. Old Cairo nearly

faces the river; but the new city is removed above a mile from it, and

approaches to the range of mountains which runs through Upper Egypt,

and abruptly breaks off here. It is undoubtedly one of the finest

cities in the east; which, from the present style of architecture that

reigns among the orientals, is but a faint commendation. The houses

are in general built of stone, and, being elevated to several stories,

would make a grand appearance, notwithstanding the inelegance of their

structure, were not the effect destroyed by the excessive narrowness

of the streets. This is one of the causes to which the ingenious Dr.

Mead ascribes the birth of the plague in this capital; but experience

evinces that it arises from _foreign and adventitious_ causes. _There

has not been a plague here these seven years_; which is rendered more

remarkable by the commencement of the Russian war at the date of its

cessation. No one can account for this; though a year seldom passed by

before without a visit from it.”[80]

[80] Irwin’s Voyage up the Red Sea p. 335.

From this it plainly appears, that, however these putrid effluvia may

concur with other circumstances in producing the plague, they are by no

means the _only_ cause; otherwise not a single year could have passed

in Cairo without a pestilence; and the very same thing we shall find

to hold good in every other, let us choose for a cause what we will.

In order to investigate this matter fully, we must now consider what

causes have been assigned by physicians for other epidemical diseases;

and here, to avoid prolixity, we shall chiefly confine ourselves to

those enumerated by Dr. Fordyce as the causes of fever; a gentleman

whose very extensive experience must give the greatest weight to his

testimony.[81] The principal causes assigned by him are,

[81] At the time of writing his treatise Dr. Fordyce informs us, that

he had been “for upwards of twenty years one of the three physicians

of St. Thomas’s Hospital (in London) whose walls have contained

nearly four thousand patients every year, where the proportion of

fevers to other diseases is much greater than the general proportion.”

1. _Infection_, or “a peculiar matter generated in the body of a man in

fever, which is carried by the atmosphere, and applied to some part of

the body of a person in health, and which causes fever to take place

in him.” That such a cause exists, he proves from observing that “of

any number of men, one half of whom go near a person ill of a fever,

and the other half do not, a greater number of the former will be

infected, in a short period afterwards, than in those who do not.” He

says he has known, in such circumstances, seven out of nine infected

with the disease. This infection is not discoverable by smell or any

other organ of sense; neither can the greatest attention to cleanliness

disarm it of its malignity. Of this the Doctor says that he has known

instances; nay, of a person going into a room where a feverish patient

was, and bringing with him the infection, which was communicated to

others in the room to which he came. He owns, however, that by allowing

the air to stagnate in which feverish patients are, the infection will

become extremely violent and fatal. This may naturally be supposed,

even without having recourse to putrid effluvia; because, independent

of these, the imperceptible infectious matter itself will undoubtedly

be accumulated in the atmosphere of the room, and act more powerfully

than it could have done had it been partly carried off and diluted by

attention to cleanliness and ventilation. He also says, that “when a

number of persons live in a small space, supposing even that they are

kept as clean as possible, it happens frequently that fever arises in

some, often in many of them. It has been in this case supposed, and is

extremely probable, that some peculiar species of matter is _produced_,

capable of producing fever, on being applied to the body.”

2. _Effluvia from putrid animal or vegetable matters._ Of this our

author seems to be less fully ascertained than of the former, as he

does not say that he has observed any instances of fevers arising from

this cause; and he concludes by observing that “either the cause of

fever, consisting of matter produced in the body of a person affected

with this disease, seems probably different from that produced by

putrefaction, or might be generated without any putrefaction taking

place.”

3. _Cold._ Our author “is not disposed to allow that sudden exposure to

cold occasions fever to take place, unless some symptom of the disease

follows immediately. If a man had been suddenly exposed to cold, and

continued in perfect health for _twenty-four_ hours, the author would

never allow that fever, or any other disease, was occasioned by it.

In this case (exposure to cold) the evidence is much stronger than in

that of infection; for the author (Dr. Fordyce) has seen many instances

where, from exposure to cold, the commencement of the attack was

instantaneous; and many are to be found in the records of medicine.”

4. _Moisture._ On this subject the Doctor observes, that the

application of water to the body is not a cause of fever, unless the

air has particles of water floating in it; in which case fever has

ensued more frequently than in other cases. Water may exist in the

atmosphere in three states. 1. In small drops suspended in it like

dust in water. 2. In vapour. In this case the transparency of the air

is not impaired, and a _chemical_ combination, as it is called, between

the air and water takes place. If the atmosphere be hot or dense, it is

capable of combining chemically with a larger proportion of water. If

therefore the atmosphere should in this manner be saturated with water,

at any particular degree of heat or density, by diminishing either of

these the vapour will be condensed, and the water reduced to the former

state of suspension in small particles. 3. Water, heated to the boiling

point, emits a steam, which combines chemically with the atmosphere,

till the latter be saturated, after which it assumes the form of small

particles; and _this last is the only state_ which has been found to

produce fever.

Moisture will also produce fever when applied to the body by wearing

wet clothes. Those which imbibe or part with heat most slowly, are

least apt to produce fevers on being heated. The warmer the atmosphere,

the more liable people are to fevers from moisture.

It has been observed, that moisture from marshes, stagnating canals, or

where the water runs very slowly, is more apt to produce fevers than

what proceeds from the sea, lakes or rapid rivers. “This (says the

Doctor) has given occasion to suppose that some other vapours proceed

from such marshes beside water, and produce the disease. It certainly

often happens, that a considerable degree of putrefaction takes place

in marshy grounds, and more especially in warm climates; but it is

by no means to be concluded that moisture in the atmosphere always

produces fever in consequence of putrefaction. Putrefaction can only

take place in vegetable or animal substances. If water therefore, not

impregnated with either, should be in such a situation as to produce

moisture in the atmosphere, no putrefaction can take place; therefore,

if fevers ensue, they are certainly in consequence of moisture, not

putrefaction. Many instances of this may be brought, as in the war

which took place in Flanders, between the tenth and eleventh year of

the present century, an army encamped upon sandy ground, in which

water was found in digging less than a foot deep, and occasioned a

great moisture in the air, which produced in a few days numbers of

fevers, although the army was perfectly healthy before, and no more

fevers were produced on shifting their ground. There are a vast many

other instances of the same thing having taken place. Besides, fever

has often arisen immediately in persons sitting in rooms, the floors of

which had been just moistened with pure water.”

5. _Certain kinds of food._ On this Dr. Fordyce observes, that, though

food of difficult digestion undoubtedly produces a number of diseases,

he has never seen it productive of fever excepting once. Dr. Girtanner

relates, that the emperor of Germany, having forced a number of his

subjects to serve as soldiers, and sent them into an unwholsome part

of Walachia, where he fed them with a kind of paste made of bread and

water instead of meat, many of them died of the scurvy. The Doctor,

however, does not ascribe this to any positive cause, but to three

_negatives_, viz. the abstraction of the stimulus of _nutriment_, by

feeding on the paste just mentioned; of the stimulus of _oxygen_ in the

corrupted atmosphere of Walachia; and lastly of the _nervous stimulus_,

the most powerful of all; the greatest part being engaged by force

against their will. This corroborates what Dr. Fordyce has said, that

bad food is very seldom the cause of fever; for among so many, who used

the _imperial_ paste just mentioned, some would certainly have been

affected by fevers, had it been capable of producing them; but, as it

did not, it is most evident that the deficiency of _stimuli_ is not the

cause of fever.

6. _Passions of the mind._ These are looked upon by Dr. Fordyce to

be among the less frequent causes of fever, though it is certain

that they have been productive of multitudes of diseases, and even

of sudden deaths; and Dr. Falconer, in his Prize Dissertation,

ascribes to the passions very considerable effects in fevers, and

even in the plague itself. “Contagious fevers (says he) afford

strong instances of the influence of mental affections, both as

prophylactics and remedies. The plague is a remarkable example, and

the same reasoning extends to other disorders of a febrile, contagious

nature. Fear, it is well observed by Dr. Cullen, by weakening the

body, and thereby increasing its irritability, is one of the causes

which, concurring with contagion, render it more certainly active,

which he ascribes to its weakening effect on the body, by which its

irritability is increased. Against this therefore he directs the mind

to be particularly fortified, which is best done by giving people a

favourable idea of the power of preservative means, and by destroying

the opinion of the incurable nature of the disorder, by occupying the

mind with business or labour, and by avoiding all objects of fear, as

funerals, passing-bells, and any notice of the death of particular

friends. Even charms might be used with good effect, could we promote

a strong prepossession of their efficacy, either by the confidence

they inspire, or by their engrossing the attention of the mind.

It is no less certain, that a studious regard to promote hope and

confidence in recovery, is equally necessary for the cure as for the

prevention of such disorders. We know that contagious fevers have a

peculiar tendency to diminish the energy of the brain, and of course to

debilitate the whole system; and that this is especially the case with

the plague, which produces the most considerable effects in weakening

the nervous[82] system or moving powers, and in disposing the fluids

to a general putrescency; and Dr. Cullen is of opinion that to these

circumstances, as the proximate causes of the plague, regard should

chiefly be had, both for the prevention and cure of this disorder.

It must therefore be highly necessary, during the course of this

disease, to attend to the support of the spirits, as on these the vital

principles greatly depend; and they can by no means be so effectually

kept up as by inspiring a confidence of recovery.”

[82] “An intense head-ach, uncommon giddiness, and a sudden loss of

strength, were the first complaints of those who were seized with

this distemper.”

(Russel on the Plague at Aleppo, p. 230.)

Dr. Zimmerman presents us with a great number of examples of the

influence of the passions in producing diseases, or death itself; some

of the most remarkable of which follow. “All the passions (says he)

when carried to excess, bring on very formidable diseases. Sometimes

they occasion death, or bring us at least into imminent danger. The

most reputable physicians agree in opinion that terror may occasion

apoplexy, and death; and indeed they consider apoplexy as the most

common effect of violent passion. Without being carried to excess, a

passion will sometimes occasion a difficulty of breathing, together

with a sense of stricture in the breast, and an hesitation to speak;

the tongue remaining as it were immoveably fixed on the palate.

Hysterical and hypocondriacal affections are sometimes the effects

of grief in the most healthy people. Joy is much more dangerous to

life than sudden grief. Sophocles died through joy at being crowned

on account of a tragedy he had composed in his old age. The famous

Fouquet died on being told that Louis XIV had restored him to

liberty. The niece of the celebrated Leibnitz, not suspecting that a

philosopher would hoard up treasure, died suddenly on finding under

her uncle’s bed a box containing sixty thousand ducats. Violent anger

has sometimes produced hæmorrhages and subcutaneous extravasations;

or, some vessel of the brain being ruptured by these transports, a

fatal apoplexy has taken place. There have been instances of excessive

anger being succeeded by epilepsy, colic, or a violent degree of

fever. Sometimes it has occasioned an increased flow of bile. In some

this produces vomiting; in others it goes off downwards, and causes

diarrhœa; or being retained, from a stricture of the gall-ducts, will

perhaps be absorbed, and occasion jaundice. In cases where anger has

been succeeded by extreme grief, obstructions have taken place in the

liver. The effects of terror are similar to those of anger, but in

general more violent. Sometimes excessive terror seems to give to men

a preternatural strength, as is the case with madmen and drowning

persons. In some cases it has not only excited immediate convulsions,

but caused them to return periodically. Fear has been said to make the

hair stand upright, and to contract the pores from which the hairs

issue in the same manner as cold does. There are instances in authors

even of the colour of the hair being changed by excessive fright.

Philip V died suddenly on being told that the Spaniards had been

defeated, and, on opening him, his heart was found ruptured. Timid

people are more liable than others to fall sick. A firmness of mind is

one of the best preservations against contagion. Willis has very well

observed, that they who fear the small-pox the most are generally the

first to be attacked with it. Cheyne assures us that fear is extremely

prejudicial in all epidemical diseases. Dr. Rogers remarks, that fear

constantly increases the ravages of a contagious disease. Rivinus

attributed the propagation of the plague at Leipsic wholly to fear. The

French physicians, who wrote on the plague at Marseilles, went so far

as to deny its being contagious, and ascribed its propagation chiefly

to fear.”

As for the cause of the plague itself, Dr. Fordyce supposes it to be

produced by an infection of a particular kind. That which takes place

in Syria and Egypt, he says, has only been clearly described by Dr.

Russel; and it cannot be gathered from the accounts whether this may

be originally produced without having been propagated as the first

class of infections above mentioned are. “That disease (says he) called

the plague, which ravaged this country (England) on considering the

histories of the disease, seems to have been a _fever_,[83] produced by

infections of the first class which have been enumerated.” Dr. Moore

has given an account of the origin of a plague, which, if it could

be depended upon, would decide the question concerning the origin of

this dreadful distemper without previous infection. This passage is

extracted from the History of the Royal Medical Society for the years

1777 and 1778. “Dr. Mitchell, physician to the hospital at Smyrna,

appears, according to a memoir of which he is the author, sent by M. le

Baron de Tott to the Medical Society to believe in the spontaneity of

the plague (or that it arises of itself without any predisposing cause

in the body) for proof of which he cites the following circumstance:

A solitary shepherd, having no communication with any body, fell sick

while he was tending his flocks; he went into an inhabited part, where

he communicated the plague with which he found that he was attacked.

This circumstance would prove much, if it was certain that the shepherd

had no communication with others; if it were known how long, and with

what precaution, he had been secluded from company: but the proofs of

these are too difficult to be established to allow of any conclusion

to be drawn from the fact. We are obliged therefore to acknowledge [it

to be a doubtful matter] whether it is in fact a country that is the

cradle of the plague; what country this is, supposing that such an one

exists; or, finally, whether it sometimes appears spontaneously, and

whether the first whom it attacks becomes the focus from whence it

emanates.”

[83] This is expressly denied by Dr. Hodges, who had innumerable

opportunities of seeing the distemper.

Dr. Fordyce, in treating of the origin of fever, seems inclined to

think that it may arise without any predisposing cause; and after

having enumerated the various causes already mentioned, and fully

considered them, gives it as his opinion, that “there must undoubtedly

be other causes than those which give occasion to the disease, but

which are at present _totally unknown_.” In like manner Dr. Moore,

speaking of the nervous fever, sums up what may be known concerning

the cause of it in the following words. “Upon the whole, we know that

people of delicate, exhausted and sickly constitutions, and those whose

minds are saddened by depressing passions, are greatly predisposed

to this disease, the immediate seeds of which, we also know, may be

generated in places where human effluvia are collected and confined.

And this is the most essential part of our knowledge respecting

the cause of this disease; and even this little is disturbed with

uncertainty: for we sometimes meet with instances of people of _robust

constitutions_, who are seized with the disease in all its malignity,

when they are under no _depressing passion_, when the disease is _not

epidemic_, to whom we cannot trace it from any place where the human

effluvia could be confined in any uncommon degree, or from any person

in the disease, of which perhaps there is no other person ill in the

neighbourhood for several miles round; and, in short, when we cannot

connect it with any of the causes supposed to be the sources of the

distemper. On extraordinary occasions of this kind we have nothing

for it but to suppose that, notwithstanding the apparent vigour of

the patient, his body has been peculiarly predisposed to catch the

infection, and that some contagion, not forcible enough to infect any

other person, has by some means, unobserved, been conveyed to him; or,

if so many suppositions displease, we may suppose at once that there

is in some cases a source of this fever which has not been suspected.

For, although the numerous observations that have been made give us the

strongest reason to think that human effluvia produce this disease,

we have no right to infer that it cannot arise also from some other

source.”

To the same purpose I subjoin the very respectable opinion and

testimony of Dr. Patrick Russel. “In some epidemical distempers, the

sudden alternations of the air have constant and manifest influence; in

others, though the influence of the air must be equally admitted, it

seems not to depend on sensible alteration or succession in the common

properties of the atmosphere, but on some _inexplicable_ combination,

some _occult, new, unknown_ quality. Amongst epidemics of this last

kind must be reckoned the plague.... Should ever that state of the

air, without the concurrence of which the contagion of the pestilence

never spreads, or ceases to act, be discovered, and ascertained by

unequivocal marks, the dread of the plague, universally prevalent,

would be greatly diminished; more effectual means of preservation would

be found out, and the application of them might safely be limited to

certain seasons.

“Experience in Turky, where, generally, no precautions are taken in

the times of pestilence, clearly evinces, that, in a certain state

of the air, a communication with infected places may subsist without

any material consequence. The return of the plague at Aleppo happens

at irregular periods; the intervals are of considerable, but unequal,

length; and in those the commerce with Egypt, Constantinople and

Smyrna remains uninterrupted. In the intervals between 1744 1760, and

from 1762 to 1780, the plague raged several times in the places now

mentioned, without affecting Aleppo; and even in two or three years

subsequent to 1762, though it was at Marash, as well as other places

not far distant, with which Aleppo has continual intercourse, no

instances were discovered of communicated infection: if such happened,

they must have escaped my utmost vigilance; and the daily exercise of

my profession led me to be very much among the natives of all ranks. At

the same time I have reason to suspect that infected families from some

of those places took refuge in Aleppo; and I know, with certainty, that

not only some merchants of that city, who happened to be at Marash when

the plague broke out there, returned to their families in the summer of

1763, but that caravans of various merchandise arrived in the course of

the same summer.

“I consider it therefore as an established fact in the Levant, that

commerce and intercourse with infected towns is sometimes attended

with no bad consequence. The same thing may perhaps be asserted,

without restriction, of all countries; but till the signs indicating

a pestilential constitution be ascertained, no particular year can

be declared exempt from danger. Predictions founded on _planetary

conjunctions_ have been long exploded; and signs derived from the known

properties and alterations of the air, are almost equally fallacious.

The seasons concomitant with plagues in England, as well as elsewhere,

have been very dissimilar; and the same visible concurrence, usually

deemed pestilential, has often, in the revolution of years, been

observed to return, in various countries, without producing the dreaded

consequences. Upon the whole, from all I have been able to collect,

the pestilential constitution seems hitherto to be known only from its

effects; _neither its approach nor its retreat can be predicted and its

nature remains wrapped up in_ MYSTERIOUS DARKNESS.”

Having thus seen, that, of the causes commonly assigned for epidemical

diseases, not one can be accounted certain and determinate, it now

remains to consider one more, and that is

_Contagion._ Though this has been generally accounted the same with

_infection_, yet by some it has been reckoned otherwise; and indeed

there seems to be a necessity for such a distinction; for, though we

should prove, ever so clearly, that a disease once communicated to one

person should from that person be communicated to another, yet the

difficulty is to know from whence the first person had it. This source,

if any such can be found, is what we may with the greatest propriety

distinguish by the name of _contagion_, and is the sense in which it

shall for the future be used in this treatise, the matter communicated

from one person to another being always called _infection_. This

indeed differs from what many celebrated physicians have said upon the

subject; but the distinction certainly must exist. Dr. Cullen speaks

rather indistinctly upon the subject. “We have supposed that _miasmata_

are the cause of intermittents, and _contagions_ the cause of continued

fevers, strictly so named; but we cannot with propriety employ these

general terms. The notion of _contagion_ properly implies a matter

arising from the body under disease, _miasma_, a matter arising from

other substances. But, as the cause of continued fevers may arise

from other substances than the human body, and may in such cases be

called a _miasma_, and, as other miasmata also may produce contagious

disorders, it will be proper to distinguish the cause of fevers by

using the terms _marsh_, or _human_ effluvia, rather than the general

ones _miasma_, or _contagion_.”

From this it is not very easy to determine what the Doctor means when

he speaks of _specific contagion_ as the cause of the plague. Dr.

Russel plainly ascribes it to human effluvia. “The plague (says he) is

a contagious disease; that is, an _emanation from a body diseased_,

passing into one which is sound, produces, in time, the same disease,”

&c. There must, however, undoubtedly have been something originally

distinct from the human body which gave rise at least to the _first_

plague that was in the world; and some plagues recorded in history are

said to have arisen in this way. Thus, Ammianus Marcellinus says that

the plague which broke out in the Roman army in the time of Marcus

Aurelius arose from a pestilential vapour confined in a golden coffer

dedicated to Apollo. Upon opening this, the _contagion_ diffused

itself all around, and the _infection_ spreading from one to another,

produced an almost universal pestilence. Ammianus indeed is the only

historian who relates this; another account of its origin is given,

p. 14, but whether we believe the account of Ammianus or not (which

indeed does not appear probable) it is sufficient to show what were

the received opinions at the time. In like manner every one has heard

of pestilential effluvia breaking out from the earth, from graves,

&c. so that we certainly look upon this doctrine of _contagion_ as

the cause of diseases to have been pretty generally received. We are

also informed by Dr. Mead, from M. Villani, who wrote the history of

those times, that the great plague of 1346 began in China, where,

according to the report of some Genoese sailors, it was occasioned

by a great ball of fire that either burst out of the earth, or fell

down from heaven. This is thought incredible by Dr. Mead, and no doubt

is so, but it shows the general opinion, that the original cause or

_contagion_ which produces a plague is distinct from the _infection_

which is afterwards communicated from one to another. In the French

Encyclopedie, we have this account of the ball of fire, or fiery

vapour, without any comment.

As to the opinion of pestilential vapours arising out of the earth,

though we are assured that people have been suddenly killed by

explosions, probably of the electrical kind, or by lightning issuing

from under their feet, yet we are not furnished with any well

authenticated accounts of a _plague_ having arisen from any such cause.

About 19 years ago a violent fever raged epidemically through a small

district in the north of Scotland, which was said to have originated

in the following manner. Some young men having heard that a certain

place in their neighbourhood had, in the time of a plague been a

burial ground, took into their heads to dig into it. They did so, and

one of them immediately fell sick, but recovered. The father of two

of the young men, exceedingly displeased at the conduct of his sons,

and apprehensive of the consequences, filled up the hole they had dug

in the ground, soon after which he fell sick and died, and the fever

continued to rage in the neighbourhood for some time. The mother of

another of the parties concerned also died, and boils broke out on

various parts of the bodies of the sick. This was the account given in

some of the news-papers of the time, and had the matter been thoroughly

investigated and attested, would have been decisive in favour of

pestilential contagion being capable of taking up its residence in the

earth. As it stands at present, it can only draw our attention to what

may happen in another case, should any similar one occur.[84]

[84] Though the writer of this Treatise was not at that time on the

spot where this event took place, yet he has as good evidence as any

one can have of what has not fallen under his immediate inspection,

that these graves were opened, that the father of one of the young

men died; and the mother of another, and one of the young men himself

was taken ill with the eruption of boils on some parts of his body;

but whether there was any person previously affected with fever in

the neighbourhood from whom it might have been derived, or any thing

which might have strongly predisposed those people to it, is unknown.

It is indeed no easy matter to discover who was the first person

affected with an epidemic, as no body chooses to own that either

they, or any of their relatives were the authors of mischief, however

involuntary, to the community. M. Chaptal, however, in his Elements

of Chemistry, has some curious, as well as useful observations on the

propriety of burying bodies in a sufficient space and at a sufficient

depth; and on the accidents which may arise from opening vaults

and burying grounds. An instance of this he gives of the ground of

a church in Paris being dug up, which emitted a nauseous vapour,

affecting several people in the neighbourhood.

From M. Chaptal’s observations it appears, that bodies do not soon

dissolve in such a manner as to emit no disagreeable or noxious

effluvia, when buried. M. Becher, he says, “had the courage to make

observations during the course of a year upon the decomposition of

a carcase in the open air. The first vapour which rises, he says,

is subtle and nauseous: some days after, it has a certain sour and

penetrating smell. After the first weeks the skin becomes covered

with a down, and appears yellowish; greenish spots are formed in

various places, which afterwards become livid and black; a thick

glosey or mouldy substance then covers the greatest part of the

body: the spots open and emit a sanies.” In such as are buried the

decomposition is much more slow; our author thinks _four times_ at

least. According to M. Petit, a body buried at the depth of four feet

is not decomposed in less than three years, and, at a greater depth

the decomposition is still more slow. This decomposition is favoured

by the presence of water, and likewise by some kinds of earth more

than others. It has been proved by Lemery, Geoffroy, and others, that

argillaceous earths have very little effect in this way: porous and

light earths much more: the roots of vegetables also by absorbing

the putrid effluvia contribute greatly to the final decomposition of

bodies buried in places exposed to the open air; but in churches and

other covered places the case is vastly different. “Here, says our

author, is neither water nor vegetation; and consequently no cause

which can carry away, dissolve or change the nature of the animal

fluids: and I cannot but applaud the wisdom of government which has

prohibited the burying in churches; a practice which was once a

subject of horror and infection.

“The decomposition of a body in the bowels of the earth can never

be dangerous, provided it be buried at a sufficient depth, and that

the grave be not opened before its complete dissolution. The depth

of the grave ought to be such that the external air cannot penetrate

it; that the juices with which the earth is impregnated may not

be conveyed to its surface; and that the exhalations, vapours, or

gases, which are developed or formed by decomposition, should not be

capable of forcing the earth covering which detains them. The nature

of the earth in which the grave is dug, influences all its effects.

If the stratum which covers the body be argillaceous, the depth of

the grave may be less, as this earth difficultly admits a passage

to gas and vapour; but, in general, it is admitted to be necessary

that bodies should be buried at the depth of five feet to prevent all

these unhappy accidents. It is likewise necessary to attend to the

circumstance, that a grave ought not to be opened before the complete

decomposition of the body. The term of decomposition is various;

according to M. Petit of three years in graves of four feet, and

four years in those of six feet. The pernicious custom which allows

a single grave to families more or less numerous, ought therefore to

be suppressed; for, in this case the same grave may be opened before

the time prescribed. It is likewise necessary, to prohibit burying in

vaults, or even in coffins.”

With regard to epidemics occasioned by the action of electricity, we

cannot indeed produce any instance; but we have one of a distemper

more dreadful than even the plague itself; and that is of a person

suddenly struck by an electric flash (generated either in his own body,

or in the room where he was) and by this stroke reduced to a most

deplorable condition, which soon ended in death. The account stands

on the authority of Mr. Joseph Battaglia, surgeon at Ponte Bosio, who

transmitted it to Florence, and is as follows.

“Don G. Maria Bertholi, a priest residing at mount Valere in the

district of Livizzano, went to the fair of Filetto, on account of some

business which he had to transact, and after spending the whole day

in going about through the neighbouring country, in order to execute

commissions, in the evening he walked towards Fenille, and stopped at

the house of one of his brothers-in-law, who resided there. No sooner

had he arrived, than he desired to be conducted to his apartment,

where he put a handkerchief between his shoulders and his shirt,

and, when every body retired, he began to repeat his breviary. A few

minutes after, a loud noise was heard in Mr. Bertholi’s chamber; and

his cries having alarmed the family, they hastened to the spot,

where they found him extended on the floor, and surrounded by a faint

flame, which retired to a greater distance in proportion as it was

approached, and at length disappeared entirely. Having conveyed him to

bed, such assistance as seemed necessary was given him. Next morning

I was called, and after examining the patient carefully, I found that

the teguments of the right arm were almost entirely detached from the

flesh, and hanging loose, as well as the skin of the lower part of

it. In the space contained between the shoulders and the thigh, the

teguments were as much injured as those of the right arm. The first

thing, therefore, to be done, was to take away those pieces of skin;

and, perceiving that a mortification was begun in that part of the

right hand which had received the greatest hurt, I scarified it without

loss of time; but notwithstanding this precaution, I found it next day,

as I had suspected the preceding evening, entirely sphacelous. On my

third visit, all the other wounded parts appeared to be in the same

condition. The patient complained of an ardent thirst, and was agitated

with dreadful convulsions. He voided by stool bilious putrid matter,

and was distressed by a continual vomiting, accompanied with a violent

fever and delirium. At length the fourth day after a comatose sleep of

two hours, he expired. During my last visit, while he was sunk in the

lethargic sleep of which I have spoken, I observed with astonishment,

that putrefaction had already made so great progress, that his body

exhaled an insupportable smell. I saw the worms which issued from it

crawling on the bed, and the nails of his fingers drop of themselves;

so that I thought it needless to attempt any thing farther, while

he was in this deplorable condition. Having taken care to get every

possible information from the patient himself, respecting what had

happened to him, he told me, that he had felt a stroke, as if somebody

had given him a blow over the right arm, with a large club, and that at

the same time, he had seen a spark of fire attach itself to his shirt,

which in a moment was reduced to ashes, though the fire did not in the

least injure the wrist-bands. The handkerchief which he had placed upon

his shoulders, between his shirt and his skin, was perfectly entire,

without the least appearance of burning, his drawers were untouched,

but his night-cap was destroyed, though a single hair of his head was

not hurt. That this flame under the form of elementary fire, burnt the

skin, reduced the shirt to ashes, and entirely consumed the night-cap,

without in the least touching the hair, is a fact which I affirm to be

true: besides, every symptom that appeared on the body of the deceased,

announced severe burning. The night was calm, and the circumambient air

very pure: no bituminous smell could be perceived in the chamber, nor

was there the least trace of fire or of smoke. A lamp, however, which

had been full of oil, was found dry, and the wick almost in ashes. We

cannot reasonably suppose this fatal accident to have been occasioned

by any external cause; and I have no doubt that if Maffei were still

alive he would take advantage of it to support an opinion which he

entertained, that lightning is sometimes kindled in the human body and

destroys it.”

Another account, to the same purpose, is given in Mr. Battaglia’s

paper. “On the 21st of April, 1781, the first battalion of the brigade

of Savoy set out from Tortona, in order to go to Arti, when the weather

was excessively hot. On the 22d, having made rather a forced march, the

soldiers suffered a great deal from the ardour of the sun, so that, at

the village of Serre, where they halted, one of them, named Bocquet, a

man of twenty-five years of age, whose skin being very hard and thick

had not perspired, sent forth a loud cry, which seemed to announce

some extraordinary commotion, and instantly fell down. Mr. Bianet,

surgeon major to the regiment, found the patient in convulsions.

When he was carried to the hospital the upper part of his body, to

the thighs, appeared to be withered and black, and in a gangrenous

state. Mr. Bianet employed scarifications, but without effect; it was

impossible to make him swallow any thing; and it was found necessary to

abandon him to his dismal fate. His body soon exhaled a putrid smell,

and he died at the end of five hours. That his disorder might not be

communicated to others, he was interred together with his clothes. Upon

inquiry, after his death, it was found that this man was addicted to

the constant use of spiritous liquors, and that he had even drank of

them to excess upon the march.”

Other instances there are, still more terrible, of people actually

taking fire and being consumed to ashes by some internal cause; but, as

nobody was present either at the beginning or during the continuance of

these extraordinary inflammations, nothing certain can be said about

them. That such things, however, have happened, is certain, of which

one of the most remarkable instances is that of Signora Corn. Zangari,

an Italian lady. She retired to her chamber in the evening somewhat

indisposed, and in the morning was found in the middle of the room

reduced to ashes, all except her face, legs, skull and three fingers.

The stockings and shoes she had on were not burnt in the least. The

ashes were light, and on pressing them between the fingers vanished,

leaving behind a gross, stinking moisture, with which the floor was

smeared; the walls and furniture of the room being covered with a moist

cineritious soot, which had not only stained the linen in the chests,

but had penetrated into the closet, as well as into the room overhead,

the walls of which were moistened with the same viscous humour. This

lady had been accustomed to use a bath of camphorated spirit of wine

when indisposed.

Dr. Zimmerman, from the 64th volume of the Philosophical Transactions,

relates the case of a poor woman who perished in this miserable manner

at Coventry in England in the year 1772. “She fell out of bed, and was

found next morning burnt to death, though the fire in the grate had

been small, and the furniture in the room had suffered but little.

Except one thigh and leg, there were not the least remains of any skin,

vessels or viscera; and the greater part of the bones were completely

calcined, and covered with a whitish efflorescence.”

On these unfortunate people it has been observed that they were

generally intemperate in the use of spiritous liquors. Of the poor

woman at Coventry, whose case has been just now related, it is said,

that she had been in the practice of drinking from half a pint to a

quart of rum every day, and this she continued, notwithstanding her

being affected with jaundice and other complaints. Mr. Wilmer, who

communicated this case to the Royal Society, concludes it with these

words: “That her solids and fluids were rendered inflammable by the

immense quantity of spiritous liquors she had drank, and when she was

set fire to she was probably soon reduced to ashes.”

On other cases of a similar nature it has been remarked, that the

miserable sufferers were “for the most part advanced in years,

remarkably fat, and had been much addicted to the use of spiritous

liquors, either in their drink, or applied in friction to the body;

whence it has been concluded that these people perished by their whole

substance spontaneously taking fire, the principal seat of which had

been the entrails, or the epigastric viscera; and that the exciting

cause was naturally found in the phlogiston of the humours, called

forth by that of the spiritous liquors combined with them.”[85] But

solutions of this kind cannot by any means be admitted. We have not

the smallest reason to think that either the solid or fluid parts of

the bodies of hard drinkers are more inflammable than those of other

people; neither is it credible that any person could live with his body

in such a state. Besides, the most inflammable bodies will not begin to

burn unless fire actually be applied to them, while others much less

inflammable to appearance, will yet take fire spontaneously. Thus, even

spiritous liquors themselves, though they flame violently when thrown

into a fire, or when a burning body is applied to them, yet there

is not an instance of such liquors taking fire of themselves; nay,

they cannot even be set on fire by pouring them upon a red-hot iron,

while, on the other hand, heaps of wet vegetables, which we should

think scarce at all inflammable, do yet very frequently take fire

spontaneously. The author lately quoted, however, justly observes that

M. Bartholi, the unfortunate priest above mentioned was plainly struck

first by electricity from without, a spark of fire attaching itself to

his shirt, and a faint flame surrounding his body; so that the fire did

not seem to have been generated in his body, but in the atmosphere.

There are instances of people being surrounded with these luminous

appearances without being hurt; particularly of a woman at Milan, whose

bed was surrounded with a light of this kind. Mr. Loammi Baldwin, of

this country, was also surrounded by an electric light, while raising

a kite in the time of a thunder storm, and Dr. Priestley makes mention

of a gentleman, who, after having worked an electric machine for a

long time in a small room, perceived, on leaving it, a luminous vapour

following him. But the instances most to our present purpose are some

recorded in the Philosophical Transactions, of luminous vapours coming

from the sea, attaching themselves to corn-stacks, and setting fire

to them. One of this kind is particularly mentioned in Lowthorp’s

Abridgement of the Transactions, as having taken place in Ireland,

coming repeatedly from the sea, and setting fire to corn and hay, so

that the people were greatly alarmed. At last they found that it might

be driven off by making a great noise, and that it would avoid any

sharp-pointed iron instrument. Had such a vapour attached itself to a

human body, it is possible that it might have set fire to it as well as

to the stack of corn or hay. Whether these accounts render the story of

the Genoese sailors concerning the ball of fire occasioning the plague

of 1346 more credible, we leave the reader to judge. They certainly

show, however, that the electric fluid will sometimes interfere with

the human body in a very terrible manner, producing, where it does not

kill instantaneously, symptoms equal to those of the very worst plague,

as in the case of the priest and soldier above mentioned.

[85] American Museum vol. xi, p. 148.

Another hypothesis concerning the origin of pestilential diseases is

that of swarms of little animals invisibly existing in the atmosphere;

which, being taken into the body by the breath, are supposed to corrupt

or otherwise vitiate the blood and other parts of the body, as we

see in the plague and other epidemic disorders. This hypothesis, so

generally exploded, and so apparently improbable, seems to receive

some support from a discovery of an insect made by Mr. Henry Baker, F.

R. S. and published in his work entitled “The Microscope made Easy.”

He called it the insect with net-like arms. “It lives (says he) only

in cascades, where the water runs very swift. Some of them being kept

in a vial of water, most died in two days, and the rest, having spun

themselves transparent cases, which were fastened either to the sides

of the glass, or to pieces of grass put into it, seemed to be changed

into a kind of chrysalis; but before they assumed this form, they

altered their shape (in a manner he represents by a figure.) None of

them lived above three days; and, though fresh water was given them

two or three times a day, yet in a few hours it would stink to a degree

scarce conceivable, and that too at several yards distance, though, in

proportion to the water, all the included insects were not more than

as one to _one million, an hundred and fifty thousand_. This makes it

probable that it is necessary for them to live in a rapid stream, lest

they should be poisoned by the effluvia issuing from their own bodies,

as no doubt they were in the vial.”

From this account it is not difficult to conceive that animals, though

exceedingly small, may yet emit such poisonous effluvia as will

destroy much larger ones in their neighbourhood. It will by no means

be incredible that, had one or two such offensive animals been thrown

into a jar containing gold-fishes,[86] the whole of these beautiful

inhabitants would have perished at once. Let us suppose such a thing

to have actually happened; that a malicious person had put them in

over night, and in the morning the proprietor of the fishes finds

them all dead, and the water offensive to the last degree. He sends

for a neighbouring philosopher, who, happening to be ignorant of the

existence of such animals, endeavours to account for the phenomenon

upon some of the received principles of philosophy. How much theory

would here be wasted, and what endless disputes might ensue without

even a _possibility_ of arriving at the truth! Just so it is with

epidemic diseases. The cause is invisible, and, until it becomes

discoverable by our senses, it can never be known; for, as has already

been observed, a cause never can be known merely by its effects, unless

we have seen it, or somebody who has seen it gives us information. And

this will certainly be found to hold good in every instance, even from

the Supreme Cause himself to the diminutive insect just mentioned.

[86] The gold-fish is a small species of carp, brought originally

from China. They are adorned with the most beautiful and resplendent

colours, and are frequently kept in jars for pleasure. They subsist

entirely on the water, without any other food. This is by Dr.

Fordyce said to be the case with all fishes, provided the water be

impregnated with oxygen.

Lastly, I shall consider another _possible_ source of epidemics, which

has been hinted at by others. Allowing that infectious matter proceeds

from the body of a diseased person, as much must issue from a single

patient as is sufficient to bring the disease upon thousands, and

with regard to the small-pox and some other distempers we certainly

know that it is so. This infection is dissipated in the atmosphere,

and intimately combined with it, so that it becomes imperceptible and

harmless; but we have no reason to suppose that it is annihilated, or

cannot be re-produced in its pristine state. Water, though perfectly

dissolved, and to appearance deprived of existence in the air, may

yet be precipitated from it, and pour down upon us in deluges. What

happens in one case may happen in another. The infectious matter,

dissolved in the air, may by some natural cause be precipitated from

it, overshadowing whole regions, and, if it be not powerful enough to

produce the epidemic of itself, may certainly _predispose_ to it in

such a degree, that the slightest additional cause will bring it on.

Something indeed of this kind would seem really to be the case,

otherwise we cannot well conceive why there should be such a

distinction of diseases. Thus the infection of the small-pox is the

same all over the world. The variolous matter will never produce the

measles in any country, nor will the typhus produce a pleurisy. The

plague manifests itself to be the same distemper in all its various

degrees of malignity, though even this dreadful disease is sometimes

so mild that it does not confine the patient to his bed. There must

therefore be some certain constitution in the nature of the cause

which produces such and such diseases, as certainly as in the seed

of particular vegetables, which gives to each its proper appearance

and shape. The cause of the disease so modified we may call, with Dr.

Cullen, its _specific contagion_.

Having thus treated so largely upon contagion of different kinds, it

now remains to consider the objections that have been made to the

doctrine altogether. It is indeed surprising that in so great a length

of time, after the world hath so often and so dreadfully suffered from

the violence of plagues, the simple fact, whether it be infectious

or not, should not have been determined: nay, that it should still be

questioned by physicians of no mean reputation whether such a thing as

contagion or infection _can possibly exist_. Dr. Mosely in his treatise

on tropical diseases treats the whole doctrine of contagion with the

utmost contempt; calling it “a field for speculation, which has long

amused the pedantry of the schools, and should never be entered into by

practical writers.” Notwithstanding this, however, he doth enter into

it, and with such bad success, that in the very first paragraph he is

obliged to derive the cause of diseases from the stars! “There are some

diseases we know, (says he) which follow the changes of the atmosphere;

but there are others which make their revolutions, and visit the earth,

at uncertain periods; for which we can trace no cause, depending on

combinations, in which, perhaps, the _influence of the planets_ may

have some share.” Here we have a still wider field for speculation than

even _the schools_ have given us; for the Doctor ought to remember that

the influence of a _planet_, producing a disease, is as truly contagion

as the effluvia of a _dunghill_; and if we have a wide field to

traverse when tracing it through the earth, we have one infinitely more

extensive in pursuing it through the heavens. But we may be assured

that planetary influence _does not_ produce diseases; for, if it did,

they would in all times of pestilence overspread the face of the earth,

as the influence of the planets, if they have any, certainly does.

The arguments used by this author against _terrestrial_ contagion are,

1. “It has often happened that hundreds of men in a camp have been

seized with the dysentery, almost at the same time, after one shower of

rain, &c. People under similar circumstances must be subject to similar

diseases: and yet it often happens that dysentery begins with a few

people, and spreads itself by degrees until a multitude are affected.”

This argument rather militates against himself; for, if dysentery

or any other disease was occasioned by an _evident_ general cause

operating upon persons in similar circumstances, all of them ought to

be taken ill at once; but Dr. Mosely owns that they frequently are

not. There must, of consequence, be something _less evident_ which

determines the disease to particular persons, while the general cause

operates equally upon all. This less evident cause we call _contagion_.

2. “It is incredible that the smelling a little human blood, that

had stood some months in a phial, gave the man a dysentery mentioned

by Pringle; or that the person Forestus speaks of got the plague

by only putting his hand into an old trunk; or that the shaking an

old feather-bed, which had lain by seven years, raised a plague at

_Wratislau_, which destroyed five thousand persons in twelve weeks, as

related by Alexander Benedictus, &c.--Such things _may be true_, but,

when probability is shaken, reason always inclines to skepticism.”

Here our author most evidently contradicts himself; for in the

beginning of the paragraph he tells us that the things related are

_incredible_, and in the end of it, that they _may be true_. The

argument, if it may be so called, is mere assertion. It is _incredible_

that the smell of putrid human blood in a vial should produce the

dysentery. Why should this be more incredible than that smelling to

a charged vial should ensure an electric shock to the person who did

so? This is entirely a question respecting a matter of fact, not

of speculation. The same is the case with the rest. It is not more

incredible that, if the infection of the plague was in a trunk, a man

should get the plague by putting his hand in it, than that he should

be burnt if he put his hand into a trunk full of hot ashes. Before the

Doctor decided in such a positive manner, he ought to have proved that

no infection could be contained in a trunk; but this, though the very

point in question, he takes for granted, first telling us that the

contrary is incredible, and then that it may be true!

3. “We observe in camps and hospitals, that those people _whose dirty

employments_ subject them in a particular manner to a _depravation of

their habits_, seldom escape the present epidemic; and this gives rise

to the vulgar expression, and very incorrect notion, of _catching_

the disease. And we observe that others from the slightest deviation

from regularity lose the power by which the body resists diseases, and

they are also attacked. But these attacks are not to be attributed to

infection: for those people who keep the vital and animal powers in

uniform confederacy, by temperance and calmness of mind (for fear, by

lowering the vital energy, subjects the body to disease) nourishing

diet, proper clothing and cleanliness, and keeping a free and regular

passage for all excretions, are proof against the assaults of foul and

pestilential air. Such people seldom suffer even by the plague itself:

while all around them perish.”

The first sentence of the above paragraph is so obscurely worded,

that it is difficult to know the author’s meaning. I know not of any

lawful employment so _dirty_ that it necessarily subjects the person

who practises it to a _depravation of habit_. The next ascribes every

thing to intemperance and fear; from which, it seems, we are to infer

that none but drunkards, cowards, and dirty, naked ragamuffins, are

ever seized with epidemic diseases. But of this we are able to bring a

direct disproof. I suppose Dr. Mosely will not say that the celebrated

Prince Eugene of Savoy was either a coward or a drunkard; that he had

a _dirty employment_, wanted proper food or clothes, or was deficient

in personal cleanliness; yet, when in the marshy parts of Hungary, he

was in danger of death from an epidemic dysentery, notwithstanding

that he was so careful in respect of diet, that he had pure water

brought him every day, probably from a considerable distance. How

came he to be affected by the distemper under such circumstances,

while Count Boneval, though as an inferior officer he probably enjoyed

fewer advantages, remained free from it, taking only a small quantity

of Peruvian bark daily? It is uncertain whether the bark did really

preserve him or not; but the case of Prince Eugene plainly shows that

sobriety, temperance, valour and cleanliness are not sufficient to

ward off an epidemic disease, if people come in the way of infection.

4. “It should follow, if contagion were supported by infected bodies,

that no person should ever escape infection (as at Oxford assizes in

1577[87]) who was within the sphere of its action; and that those

who were entirely secluded from it, and free from all contiguity to

infected people, or substances, as the collegers were in the town of

Cambridge, when the plague was last in England, should be exempt from

it.

[87] Sir John Pringle, from Stowe’s Chronicle, gives the following

account of these assizes. “On the 4th, 5th and 6th days of July were

the assizes held at Oxon, where was arraigned and condemned Rowland

Jenkins, _for a seditious tongue_; at which time there arose amidst

the people such a damp, that almost all were smothered. Very few

escaped that were not taken. Here died in Oxon three hundred persons;

and sickened there, but died in other places, two hundred and odd.

“The sessions at the Old Bailey in Westminster, in 1650, proved also

fatal to many; of which Sir John also gives an account. ‘I have been

informed (says he) that, at those Sessions, about a hundred were

tried, who were all kept in close places as long as the court sat;

and that each room was but 14 feet by 11, and seven feet high. The

bail-dock is also a small room taken off one of the corners of the

court, and left open at the top: in this, during the trials, are put

some of the malefactors who have been under the closest confinement.

The hall in the Old Bailey is a room of only 30 feet square. Now

whether the air was most tainted from the bar by some prisoners then

ill of the jail distemper, or by the general uncleanliness of such

persons, is uncertain; but it is probable that both causes concurred.

And we may easily conceive how much it might have been vitiated by

the foul steams of the bail-dock, and of the two rooms opening into

the court in which the prisoners were the whole day crowded together

till they were brought out to be tried. It appeared afterwards, that

these places had not been cleaned for some years. The poisonous

quality of the air was aggravated by the heat and closeness of the

court, and by the perspirable matter of a number of people of all

sorts, penned up for the most part of the day, without breathing

the free air, or receiving any refreshment. The bench consisted of

six persons, whereof four died, together with two or three of the

counsel, one of the under sheriffs, several of the Middlesex jury,

and others present to the amount of above forty; without making

allowance for those of a lower rank, whose death may not have been

heard of; and without including any that did not sicken within a

fortnight after the sessions.’”

(Pringle’s Observations p. 329 & seq.)

“But, in opposition to this, _Rhazes_ lived 120 years, an often

practised in plagues. _Hodges_ remained in town, and attended the sick,

during the great plague in 1665. _Kaye_ was in the midst of practice in

the sweating sickness in 1551, without any inconveniency. _Procopius_

informs us, that during a terrible plague at Constantinople, in 543,

which almost destroyed the whole city, no physician nor other person

got the plague by attending, dressing or touching the sick. Yet most

of the Capuchins, the Jesuits, the Recollets, the Observantines,

the Barefooted Carmelites, the Reformed Augustines, all the Grand

Carmelites, the Grand Trinitarians, the Reformed Trinitarians, the

Monks of Loretto, of Mercy, the Dominicans, and Grand Augustines, who

kept themselves secluded in their several convents, and took every

precaution to avoid the plague, while it raged at Marseilles, perished

by it.

“There are no epidemical nor contagious diseases that attack every

person who breathes the same air, or that is in contact with the

infection, else whole regions would be depopulated. The habit must

be graduated, or adapted, for the reception of a disease. In some

constitutions of body the access is easy, in some difficult, in others

impossible. _But where the revelation of this mystery is to be found,

none can tell._”

In this, which our author seems to have designed as his _grand

argument_, it is plain that the deficiency is as great as in any of

the rest. If we suppose the plague, or any other epidemic disease, to

arise from some general cause, let that cause be _contagion_ or any

thing else, it ought to operate upon all who come within its sphere

of action, as Dr. Mosely observes of _infection_. If experience

shows that it does not, the argument will hold equally against a

constitution of the atmosphere, putrid effluvia, heat, cold, or any

thing else; and in fact the Doctor fairly gives up the point at last,

by resolving the whole into an _unrevealed_ mystery. With regard to

what he says about the plague at Marseilles getting into the convents,

of which he presents us with such a catalogue, it is impossible to

know what precautions were used, and we are assured that in Turky it

is thought necessary for the Europeans not only to guard against a

communication with their own species, but some of the brute creation

also. Cats particularly are dreaded so much, that a general massacre

of them commences among those who use precautions, the favourites

of that species must be sent to a distance, and M. Volney mentions

two merchants who had shut up their houses, and yet had the plague

imported by a cat. In short, considering that infection is supposed to

be altogether invisible and imperceptible, it is impossible to say how

it may be conveyed, or to what extent it may occasionally act when once

brought into a country. Dr. Fordyce is of opinion that the distance at

which infection may act depends on the disposition of the air at the

time; and he observes, that a difference in this respect is observable

in the odoriferous effluvia of vegetables. “If the air be loaded with

moisture, they reach to a much greater distance. Vapour arising from

a field of beans, for instance, or a putrid ditch, is sensible to the

nostrils at a greater distance if the air is moist.” He observes indeed

that this has never been verified with regard to infection; but as it

is evidently the case with putrid effluvia, which very often accompany

infection, we may reasonably conclude that it is the case with the

latter also.

Let us next take a view of what is advanced by the authors of The

Science of Life upon this subject. Mr. McLean, who puts his name to

this part, informs us of his conviction “that no general disease, which

affects a person more than once during life, can ever be communicated

by contagion;” and he defines contagion “a specific matter, generated

in a person affected with disease, and capable of communicating that

particular disease, with or without contact, to another.” It would

here be no improper question, by what means he comes to know that a

contagious disease can affect a person only _once_. But even this

question is unnecessary. Dr. Guthrie gives an account of a gentleman

who had the courage to inoculate himself for the plague, in consequence

of which he had the disease with the concomitant symptoms of buboes,

&c. Here then we see the plague communicated by “a specific matter

generated in a person affected” with the same disease, i. e. by

_contagion_, according to Mr. McLean’s own definition. The dispute

therefore might stop, as this fact seems to be decisive on the subject;

but as he has at great length insisted upon the argument last quoted

from Dr. Mosely, it seems necessary to follow him a little farther.

“If a person (says our author) be affected with any disease, it will

necessarily be communicated to every other person who comes within the

infectious distance, and _is not at the same time labouring under some

disease higher in degree_.” This proceeds upon a supposition that his

theory is absolutely perfect and infallible; which, however plain it

may appear to himself, will not probably be admitted by others without

some proof. Indeed he himself afterwards adduces some facts which

decisively overthrow it. “A child (says he) here and there is exempted

from small-pox, even though exposed to its contagion.” How comes this

to pass? The disease, we are told, is contagious, the child is exposed

to the contagion, and yet is not affected. In _all_ such cases it would

be ridiculous to suppose the subjects labouring under a disease higher

in degree than the contagion could produce. In numbers of instances of

this kind the children were evidently in good health, and yet would

perhaps be seized at an after period when no more exposed to contagion

than they had been at first.

“Small-pox, measles, and other general diseases, which occur only once

during life, never disappear, until _the whole_ of those who have been

within the infectious distance, and were not at the time labouring

under some disease higher in degree, have received the infection. As

these diseases are _very mild_, children sometimes resist the power of

contagion from the superior force of some other diseases, although they

may be so slight as to escape _common_ observation.”

In this paragraph we have the favourite maxim of our author repeated,

_twice_ indeed, without a single fact to support it. Instead of this

we find hypothesis heaped upon hypothesis, as the giants are said to

have heaped mountains upon one another in order to get up to heaven.

He first supposes that the infection of the small-pox seizes on _the

whole_ of those on whom it falls. The exceptions to this maxim he

explains by another _supposition_, viz. that the contagion of the

small-pox is counteracted by another disease. The second hypothesis

is supported by a third, and that a very extraordinary one, that

the small-pox (a disease which has destroyed innumerable multitudes)

is _very mild_; and this third by a fourth, that the diseases which

counteracted the contagion were so slight as to escape _common_

observation. It was incumbent on Mr. McLean to have pointed out some

of those diseases, and to have informed us how they came to counteract

this contagion. But it is needless to argue with one who writes

so extravagantly. Far from the mode of reasoning followed by Dr.

Fordyce, who decided from the majority of facts, our author determines

every thing by his own preconceived opinions. “That the power which

occasioned disease at the Oxford assizes (says he) was not contagious

matter, is proved by its producing diarrhœa in some, while it produced

fevers in others.” But, if it was not contagious matter, what kind of

matter was it? Or how comes our author to know that those who were

affected by the diarrhœa were not likewise affected by fever? How many

fevers are attended by diarrhœa, or how many cease when diarrhœa comes

on! It would have been equally conclusive to say that the matter was

not contagious, because some died and some recovered.

I shall only take notice of one assertion more, it being both tedious

and unnecessary to follow him through the whole. “From _every_ record

of epidemic and pestilential diseases, it would appear, that they have

their stated periods of recurrence; that these periods are such months

as are most remarkable for vicissitudes of the atmosphere; that they

become general only in those years in which these vicissitudes are

extreme; that they do not occur in seasons when the heats or colds,

however intense, are equable; nor in years when the state of the

atmosphere is tempered throughout; and that they uniformly cease with

the establishment of an equable state of the atmosphere, whether the

weather be cold or hot.... In Aleppo, according to Dr. Russel, the

Europeans regularly shut themselves up in their houses _every year_,

at some period between April and July; and the rich natives begin to

adopt the same plan, &c.... From this fact it appears, that the plague

occurs at Aleppo, in a state more or less mild, almost annually, and

_that it commences and ceases at certain known periods_. But it has

been remarked that, in its most severe state, this disease recurs only

at periods of ten years, or _thereabouts_: a regularity which cannot,

upon any known principle, be attributed to a power of such casual

application as contagious matter.”

In the beginning of this paragraph our author makes a bold appeal to

_every_ record of epidemic and pestilential disorders; but here we

may ask, Has he consulted _every_ record of these disorders? That he

has not, we may readily believe; but even those which are hinted at

seem either to have been very inaccurately consulted, or wilfully

misrepresented. To evince this I subjoin the following abstract of

what Dr. Alexander Russel says of the plague in general, with the

annotations of his brother, Dr. Patrick, taken from Russel’s Natural

History of Aleppo.

The inhabitants of Aleppo suppose that the plague visits them once in

ten years, and that it is always imported; and the most severe plagues

are thought by some to come from Damascus, while others contend that

they come from the northward. Dr. Alexander Russel thinks this popular

opinion of the return of the plague not altogether unfounded; and he

thinks it also probable that it never invades Aleppo without having

previously attacked either Damascus or Khillis, Aintab, Marash or Uufa.

He thinks that its appearance always is in one of the maritime towns of

Syria; if in Sidon, Byroot or Tripoli, Damascus is commonly the channel

by which it reaches Aleppo; but, if it shows itself first at Scanderoon

or Byass, its approach is by the way of Khillis or Aintab.

On this Dr. Patrick Russel observes, that the account of Aleppo being

visited only once in ten or twelve years is confirmed by a letter from

an English gentleman, in 1719, who had resided there for 30 years. The

dates of the plagues which Dr. Patrick had procured were, 1719, 1729

and 1733. Another began in 1742, and terminated in 1744; from which

time there was no return till 1757 or 1758, when it continued at Aleppo

till 1762, and did not entirely quit the country till 1764. The plague

of 1719 was said to come from the northward, but this appeared to want

confirmation; but all accounts agree that it raged at Tripoli, Sidon,

&c. two months before it appeared in Aleppo. Egypt was ravaged by the

plague in 1728, as was also Byass and the neighbouring parts in the

same summer; and next year it appeared at Aleppo. In 1732 it raged at

Sidon, Tripoli and Damascus; next year it seized Aleppo.

Dr. Alexander goes on to inform us, that the disease never spreads much

in winter. It advances with the spring, comes to its height in June,

declines in July, and terminates in August. “None (he says) are ever

seized with in September and October, not even in the plague of 1742,

which returned three years successively;” but Dr. Patrick says that

this was not confirmed by his experience in 1760, though he owns that

the distemper declines remarkably at that period; and the natives are

greatly inclined to have it believed that the distemper has totally

ceased, and to deceive the Europeans in this respect. The times at

which the Europeans shut up and come out of their confinement show

only the increase or decrease of the disease, but not its beginning or

ending. The plague of 1719 made terrible havoc. Europeans then shut

up about the middle of March, and kept confined till the middle of

July. In 1729 they did not shut up till the middle of May, and were

not confined above a month, the number of sick being small. In 1733

they were confined from the middle of March to the middle of July,

but the distemper was less violent than in 1719. In 1742 the time of

confinement much as in 1729. In 1743 shut up April 11, and opened the

middle of July. The plague violent, but less so than in 1733. In 1744

few shut up, the number of sick being inconsiderable. In 1760 they shut

up on the 30th of June, and continued about a month. In 1761 shut up

May 28, rode out Aug. 1, and opened completely the 10th of that month.

In 1762 they were confined from the last week in May to the first of

August. From 1762 to 1787, a larger period than usual, the city was

free from the plague. In 1787 it broke out among the Jews in the month

of April, increased in May, raged violently in June, and terminated in

July.

From these accounts it appears, as Dr. Alexander Russel informs us,

that the plague of one year differs remarkably from that of another;

but he says, that, at Aleppo, it is never attended with such scenes of

horror as have been known in European countries; for which Dr. Patrick

assigns the following reasons: 1. The markets are constantly supplied

with provisions. 2. The dread of the contagion is much less. 3. The

sick are less liable to be deserted by their attendants (but this,

according to his own observation, is not always the case) and 4. The

regular, speedy interment of the dead prevents a spectacle far from

uncommon in the European plagues, and which of all others is the most

shocking to humanity.

“Extreme heat (says Dr. Alexander) seems to check the progress of the

distemper. July is a hotter month than June, and the season wherein the

plague ceases at Aleppo is that in which the heats are most excessive.”

His experience did not confirm a popular opinion at Aleppo, and which

has likewise been adopted by many medical writers, that the moon has

any influence on the distemper. To have had the distemper once does

not secure a person against future attacks. Numbers of people who were

alive when he left Aleppo had it twice or oftener; and he had instances

of some being infected thrice in one season. Dr. Patrick Russel has

observations to the same purpose.

From this it appears, that the popular opinion at Aleppo, which Mr.

McLean wishes to establish as a certainty, is by no means so well

founded that we can build any theory upon it. The misfortune is, that,

wherever a theory is built upon any thing said to be constant and

invariable, a single failure overturns the whole. Now, in the dates

of plagues above mentioned, the variations are so great that it is

impossible to draw any certain conclusion from them. In the first

three instances of 1719, 1729 and 1733 there is indeed a coincidence of

the first two, but the last falls short by no less than _six_ years.

What then does Mr. McLean mean by his “ten years, or _thereabouts_?”

Can _thereabouts_ imply a difference of more than half? The English

gentleman’s testimony who resided 30 years in that country could extend

no further than to three plagues, and even these are not mentioned. The

fourth instance in 1742 is deficient in one year; the fifth in 1757 or

1758 exceeds by three or four years, and the sixth from 1762 to 1787 by

no less than fifteen years.

An anonymous writer in a Scots periodical publication entitled “The

Bee,” has partly adopted the above opinion, but adds others for which

he has not thought proper to adduce any authority. “It visits _most

parts of Asia_ once in ten or twelve years, and carries off an eighth

or tenth of the inhabitants. There have been plagues which have carried

off one fourth of the inhabitants. The farther east you go, the less

frequent it is--every 20th, 40th, and, even at Bassorah, every 90th

year; but then this scourge is most dreadful. The last plague at

Bassorah, which had not visited the city for 96 years, carried of

more than nine tenths of the inhabitants.”[88] It is astonishing that

people will write in such a manner as to subject themselves to endless

criticism on account of their inconsistency. The plague, this writer

says, visits _most parts_ of Asia once every ten or twelve years, and

yet it goes no farther east than Bassorah; a space scarce equivalent to

the twentieth part of Asia! Even in this small space, it varies from

ten or twelve, to twenty, forty, or even ninety years; and, to complete

the whole, instead of giving any instance of the periodical return

of the plague at an interval of _ninety_ years, we have one of its

disappearance for _ninety-six_ years!

[88] Bee, vol. xviii, p. 282.

From all this it is evident, that no dependence can be placed on such

vague accounts with regard to the periodical returns of the plague.

Even the time of shutting up the houses in Aleppo is not accurately

related, for, from the above abstract it is plain, that they are

sometimes shut up in March; while Mr. McLean would have us to believe

that it is always between April and July. It is needless to wade

through a jumble of unsupported assertions, which, being backed by no

evidence, fall to the ground of themselves. “I will _venture to assert_

(says he) that no person in perfect health ever was or _can be_ exposed

to the power of contagion, without receiving the specific disease which

that contagion produces; excepting in small-pox, measles, &c. when

the person has previously had the disease.”--How comes he to know all

this? Or, though our author ventures to assert, must we of necessity

_venture to believe_? When he ascribes the origin of epidemics, and

the plague itself, to the vicissitudes of the atmosphere, not a single

fact is adduced in support of his hypothesis. One very strange proof

indeed he brings from Dr. Rush, viz. that the latter had been informed

by a gentleman who resided in tropical countries, that, in the month

of July, several weeks before the yellow fever became general, he had

observed a peculiar and universal sallowness of complexion in the

countenances of the people of Philadelphia, such as he had seen in

those of the more southern countries before the appearance of bilious

fevers in them. Surely it is a very strange mode of argument to tell

us of the colour of people’s countenances instead of the states or

vicissitudes of the atmosphere, which we are made to believe were the

causes of that change. Another quotation is made from the same author

in which a warm, dry, stagnating air is _conjectured_ to have been

the cause of _diseases_; but he does not even quote Dr. Rush saying

that it was the cause of yellow fever, much less of all epidemic

diseases. Besides, to say that any thing is occasioned by a _state_, or

_vicissitude_ of the atmosphere, is such a vague mode of expression,

that it must either mean nothing, or be contradictory to itself. A

_state_ of the atmosphere we must suppose to mean that it continues for

some time either to be wet or dry; a _vicissitude_, when it changes

from one to the other. If an epidemic then is produced by a _state_,

it cannot also be produced by a _vicissitude_, of the atmosphere: or,

if some epidemics are produced by states, and others by vicissitudes,

we ought to be informed which produce one kind, and which another. But

throughout the whole of this dissertation we have neither distinctness

nor regularity, nor indeed any thing but assertion, supported only by

an imaginary theory.

Dismissing at length therefore these conjectural theories, let us

endeavour to deduce from certain and undoubted facts the connexion

between the state of the body, and the operations upon it of other

causes, invisible indeed to our eyes, but discoverable by our rational

faculties, and in some measure capable of being made the objects of our

senses also.

1. From the account given of the structure of the human body, it

undeniably follows, and has already been observed, that all parts of

it are so connected together, that none can suffer any very grievous

injury without affecting all the rest.

2. The life of man depends immediately on the air. From this element

the _blood_ receives heat and a vital spirit diffusing itself from the

blood along the nerves, and thence expended in the operations of life

and sensation.

3. From undoubted experiments[89] it appears, that this vital spirit

possesses in a great degree the properties of electricity, insomuch

that many suppose them to be the same. This is indeed denied by the

celebrated anatomist, Dr. Monro, but he allows that the nervous fluid

is similar to electricity, and it is certain that the electrical fluid

can affect it in such a manner that we may reasonably believe them to

be the same.

[89] Those of Galvani and others on _animal electricity_.

4. The air acts upon the blood by the _latent_ heat it contains. The

air itself is composed of something volatilised by heat. In some cases

this is evidently a terrestrial substance, as in that of inflammable

air, or hydrogen, which is formed of charcoal volatilised by heat,

with the addition of a little water. In the case of oxygen, or

dephlogisticated air, the combination seems to be the matter of heat

(which I shall hereafter distinguish by the name of the _ethereal

fluid_) with water deprived of its carbonic principle. This coincides

with the opinion of Dr. Priestley, who says that the basis of

dephlogisticated air seems to be _dephlogisticated water_. But, let

the basis be what it will, the ethereal fluid which volatilises it is

the _agent_; the basis is entirely _passive_, and only modifies or

restrains the action of the other fluid, so that it does not exert

itself except in particular cases. Fixed air, or carbonic acid, is

composed of the base of oxygen united with a certain portion of carbon,

and the whole volatilised by the ethereal fluid. Phlogisticated air,

azote, or septon, according to Dr. Priestley, consists of the basis

of dephlogisticated air along with a certain proportion of carbon

different from that which produces fixed air, volatilised by the same

agent;[90] and so we may determine concerning every other species of

air.

[90] See Medical Repository, vol. ii, No. iii.

5. In certain cases the ethereal fluid quits those substances with

which it is united: the air is then decomposed, the substance into

which the other fluid enters is heated, or rendered more fluid than

before (perhaps both) while the basis either unites itself to the

moisture of the lungs, or is thrown out by the breath. Whether in any

case the basis can pervade the membranes, and thus mix itself with the

blood, notwithstanding the positive assertions of Dr. Girtanner and

others, is very doubtful, and does not admit of any positive proof.

6. The blood, being a _fluid_, must be subject to the same laws with

other fluids. A certain quantity of _latent_ heat must be contained

in it, in order to give the degree of fluidity naturally belonging to

it. If this quantity be augmented, the fluidity will be augmented, and

the blood will become thinner; if it be diminished, the contrary will

take place; and if we suppose a great proportion of this latent heat to

be abstracted, it is not unreasonable to suppose that something like

a congelation may take place, and the blood be changed into a solid

substance of such a nature as cannot any more be made to resume its

former qualities.

7. By augmenting the sensible heat, the blood is affected in the same

manner as any other fluid; it suffers expansion, by which the vessels

are dilated in proportion, and, if this expansion and dilation be

carried to a certain length, a rupture of many of the small vessels,

and apoplexy, or some other grievous disease, may ensue.

8. By breathing certain kinds of air, the fluidity, heat and expansion

of the blood, and of consequence the dilation of the blood-vessels,

are affected. Thus, when a person breathes a quantity of the fume of

charcoal, containing much fixed air, he feels himself affected with

pain and a sensation of fulness in his head; he becomes sleepy, and,

if the quantity be sufficiently great, he falls into an apoplexy, and

dies. From dissections it appears that such as die in this manner have

the capillary vessels greatly distended, and even ruptured; the heat

of the body is vastly augmented, and even continues some time after

death. Hence it is evident, that, by breathing this kind of air, too

much _sensible_ heat is conveyed to the blood. In like manner when we

breathe the steam of water, if any quantity of that steam be condensed

in the lungs, the whole quantity of latent heat contained in that

steam discharges itself upon the lungs, and increases the sensible

heat of the body; and from this we may learn why on some occasions

our sensations should so ill correspond with the thermometer, and why

a warm air almost saturated with moisture should always appear much

hotter than a dry one, though the thermometer stand at an equal height

in both. Oxygen air seems to convey to the blood a much larger quantity

of what we have called _vital spirit_, than any other kind. Whether

this vital spirit be the same with the latent heat of the blood, we

know not; but, as this kind of air is evidently capable of supplying

the blood both with latent and sensible heat, it seems most probable,

that, by breathing a considerable proportion of it, both these kinds of

heat, as well as the vital spirit itself, will be augmented. In this

case, wherever the air naturally contains a larger quantity of oxygen

than usual, the blood ought to be mere fluid, as well as warmer, than

usual, provided there be no evident cause why it should be otherwise.

Accordingly in warm climates it is always found that the blood is

thinner and more fluid than in such as are colder; but at the same

time the temperature of the body is colder than in other countries.

Zimmerman tells us, that, “at Curassau, Europeans gradually lose their

fresh colour and vivacity: their natural heat even becomes three or

four degrees less than it was at their arrival.” The reason of this

last, however, is evidently the excessive perspiration, which is more

than sufficient to carry off the superabundant quantity of sensible

heat thrown into the body, either by the rays of the sun, or by the

superior quantity of oxygen naturally existing in the atmosphere; for

it is now found, contrary to the opinions hitherto received, that in

the warmer climates the atmosphere contains a larger proportion of

oxygen than in the more temperate.[91]

[91] That this is the case with the atmosphere at Martinico is now

determined by a letter from Dr. George Davidson to Dr. Mitchell of

New York, inserted in the Medical Repository, vol. ii, p. 279. With

equal parts of nitrous and atmospheric air there was an absorption of

67 parts out of 100; but when two parts of atmospheric air were used

to one of nitrous, the absorption was only from 52 to 58 parts; with

a mixture of iron filings and sulphur, upwards of four tenths of the

air were absorbed. These experiments were attested by a number of

medical gentlemen who were present. In a letter subjoined from Dr.

Chisholm, he says, that, having made a trial with iron filings and

sulphur, the absorption was forty parts of an hundred, or exactly

four tenths, with the eudiometer fifty-six. “It appears to me (says

Dr. Chisholm) to be a singular circumstance, that, although the

ground on which the Ordnance Hospital Hands is a perfect morass,

partially drained, yet a result almost exactly similar to that given

by the experiments made with the eudiometer at my house, should take

place, with the same instrument and in circumstances very different.

The proportion at the Ordnance Hospital, I think, has been 58 out of

100, and at your house, a situation less swampy, and nearer the sea,

it has been 67. An explanation of so singular a result, in situations

so different, is perhaps more to be wished than expected.”