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!
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.
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.