The substances capable of transmitting the contagion of the plague are various. The most efficacious in this respect, and therefore the most dangerous, are soft porous bodies, and, first among these, animal substances, such as wool, feathers, furs, hair, and silk. After these follow cot ton, flax, hemp, paper, and all manufactures into which these materials enter. It has been said that metal and wood do not convey it ; but it is suspected that foul and rusty metals, such as money, can transmit it, and that wood, unless varnished, is doubtful or suspicious. It is thought in the Levant that it is not conveyed in bread, but at Mar seilles the physicians are of a different opinion. It is said that it cannot be attached to plants, but that it does attach itself to odoriferous flowers, and that it is conveyed in this way. Dogs, cats, rats, and other domestic animals, form other modes of communication, and arc therefore destroy ed wherever proper precautions are adopted.
All these substances arc no less dangerous for the faci lity with which they part with the contagion to the human species, than for the pertinacity with which they retain it, and that for a great length of time. It has been found more than once, that ropes, bed-clothes, and other similar articles, which had been used about patients infected with the plague, have communicated the disorder even many years after they had been laid by and forgotten. It is suf ficient merely to touch such 'an object to become infected, and in fact thee are even more dangerous than the bodies of persons labouring under the disease. In this way an individual may carry the contagion in his clothes without being himself infected ; and some remarkable instances have occurred, where a person has carried infected clothes about him for a considerable time without suffering, when the casual contact of them, after being taken off, had ex cited the disease in another person.
It is an important question to ask, whether the conta gion of the plague can be communicated through the air alone. This is the question between infection and conta gion, which has been the subject of so much controversy. The most prevailing opinion, however, is, that the disease is exclusively contagious, and not infectious, or that it can only be communicated by contact. Whatever the truth may be in this case, the doctrine may become inconve nient and dangerous. There is no doubt that, if there is a free circulation of good air, a sound person does not re ceive the infection from a diseased one, provided that he keeps at a small distance. This is experienced every day in the plague countries.
But this is not the case when the air is stagnant or foul, charged with various exhalations, or with miasmata and other seeds of disease. Such air, it is undoubted, acts the part of a conductor to the poison, and produces the same effects as immediate contact would do. Thus, in the case of common typhus, the air of an ill-ventilated hospital will transmit or produce the fever, even at great distances from the bodies of the patients. In a similar manner it has been found that the same contagion, as well as that of the plague, has been propagated rapidly, and with the most pernicious effects, in low grounds, then subject to marsh miasmata, when the same people, in more elevated and in more ventilated situations, escaped.
These remarks apply to the plague considered indepen dently. That disorder has its three periods of rise, of se verity, and of decline. At the two extremes, nothing of this kind can take place. But when the disease is at its height, and the infected numerous, and when some neglect and disorder are the necessary consequence, it is more than likely that some accumulation of pestilential matter, to a dangerous degree, may take place in an air which, at all times, in narrow streets and similar situations, must be considered a favourable conductor, from the quantity of various impur ties which it contains. In these cases ven tilation is not easy. as every day's experience proves, from the smells thus accumulated in the narrow streets and close parts of all great towns in particular. This was fully proved in Vienna, when, about the height of the plague in that city, the air was so contaminated, that the disease was taken by merely passing through the streets. That it has been communicated a thousand times by the breath of a patient, there is no doubt ; and thus physicians and nurses have fallen victims to the disease in hospitals, even when the greatest precautions against contact were used.
To this foul state and consequent conducting power of the atmosphere, we must attribute the bad effects that have been experienced from burning various matters in apart ments, and lighting fires in the streets ; a practice as old as the time of Hippocrates. This practice was tried at Toulon, so that the air was thick with smoke, and the consequence of it was materially to increase the rapidity of the progress of the disease. In houses, the same con sequences have been produced by the use of perfumes and fumigations, a fashion so common in all diseases; but the utility of a pure air produced by a free ventilation is now well understood.