in Physic, the communicating, a disease from one body to another. In some diseases it is only effected by an immediate contact or touch, as in syphilis; in others it is conveyed by infected clothes; and in others it is supposed to be transmitted through the air at a considerable distance, by means of steams or effluvia arising from the sick, as in the plague and other pestilential disorders, in which case the air is said to be contagious, though this has been disputed.

No attempts which have yet been made to investigate the nature of contagion, or to ascertain the properties of contagious matter, have proved successful. But from the means which have been effectually employed either to abate its virulence or to destroy it entirely, this matter may be fairly inferred to be of a chemical nature. We have already detailed the effects of the fumes of muriatic acid in purifying the cathedral of Dijon, which were successfully used by Morveau in Pursuing this hint, no doubt, Dr Carmichael Smyth proposed the fumes of nitric acid. This was tried on board different ships at Sheerness about the year 1796; and being found to answer the purpose of destroying the contagion which then prevailed, Dr Smyth afterwards received a liberal reward from government for his discovery. These experiments were conducted on board the Union hospital ship by Mr Menzies surgeon of the Discovery, and Mr Ballan surgeon of the Union. The wards at this time were very much crowded with patients; and of 200 sick on board, 150 were in different stages of a malignant contagious fever, which made a very rapid progress, and produced very fatal effects on the attendants and ship's company.

The materials and apparatus employed in the process were the following: A quantity of fine sand, two dozen quart earthen pipkins, as many common tea-cups, some long slips of glass to be used as spatulas, a quantity of concentrated sulphuric acid, and a quantity of saltpetre (nitrate of potash).

The process was conducted in the following manner: All the ports and scuttles were shut up; the sand, previously heated in iron pots, was scooped out into the pipkins with an iron ladle; and in this heated sand, in each pipkin, a small tea-cup was immersed, containing about half an ounce of sulphuric acid, to which, after it had acquired a proper degree of heat, an equal quantity of nitrate of potash in powder was gradually added, and the mixture stirred with a glass spatula till the vapour arose from it in considerable quantity. The pipkins were then carried through the wards by the nurses and convalescents, who kept walking about with them in their hands, occasionally putting them under the cradles of the sick, and in every corner where any foul air was suspected to lodge. In this manner they continued fumigating, until the whole space between decks, fore and aft, was filled with the vapour, which appeared like a thick haze.

The vapour at first excited coughing among the patients, which gradually ceased as it became more generally diffused through the wards: part of this effect, however, was to be attributed to the inattention of those who carried the pipkins, in putting them too near the faces of the sick; which caused them to inhale the strong vapour as it immediately issued from the cups.

The body-clothes and bed clothes of the sick were, as much as possible, exposed to the nitrous vapour during the fumigation; and all the foul linen removed from them was immediately immersed in a tub of cold water, afterwards carried on deck, rinsed out, and hung up till nearly dry, and then fumigated before it was taken to the wash-house: a precaution extremely necessary in every case of infectious disorder. Proper attention was also paid to cleanliness and ventilation.

Three hours were at first found necessary to fumigate the ship. In about an hour after, the vapour having entirely subsided, the ports and scuttles were thrown open for the admission of fresh air. It could plainly be perceived that the air of the hospital was greatly sweetened even after this first fumigation. The process was repeated again next morning; and the people employed, being now more expert, finished the whole in about an hour's time. In an hour afterwards, the vapour having entirely subsided, the fresh air was freely admitted into the hospital as before. Fewer pipkins were employed for the evening fumigations than for those of the mornings, as the fresh air could not be admitted so freely after the former as the latter.

The pleasing and immediate effect of the fumigation in destroying the offensive and disagreeable smell, arising from so many sick crowded together, was now very perceptible, even to the attendants; the consequence of which was, that they began to place some degree of confidence in its efficacy, and approached the cradles of the infected with less dread of being attacked with the disorder; thus the sick were better attended, and the duty of the hospital was more regularly and more cheerfully performed.

From the 26th of November 1795, when the fumigation was first resorted to, till the 25th of December, not a person on board was attacked with the fever, though, in the three months preceding, more than one-third of all the people in the ship had been seized with the distemper, and of these more than one in four were carried off by it; and the probability is, that the sickness and mortality would have gone on, increasing in proportion to the diffusion of the contagion, and to the increasing dependency of the people, who considered themselves as so many devoted victims.

The advantage of the fumigation was not felt by the ship's company and attendants alone, whom it preserved from the baneful effects of the fever: the sick and convalescents derived almost an equal benefit from it. The symptoms of the disease became milder, and lost much of their malignant appearance; and the advantage of a pure and sweet air to convalescents must be obvious.

Great confidence is always dangerous. It proved so on the present occasion. On the 17th of December they imagined themselves so secure, that they discontinued the custom of fumigating morning and evening, thinking that once a day was sufficient. On the 25th, one of the nurses suffered a slight attack; and on the 26th a marine, who, for a week before, had been in a state of intoxication, was seized with the fever, and died. These two accidents gave immediate alarm: they returned again to the practice of fumigating twice a-day; and from that time to the extermination of the disorder, there was not an instance of a person suffering from contagion on board the ship.

The success of the experiment was not confined to the Union: the power of the nitrous vapour to destroy contagion was equally displayed on board some Russian ships then in the Downs. The safety, too, with which it may be employed, in any situation, without inconvenience or risk of fire, is another great recommendation in its favour.

It will not be difficult from this description to employ this kind of fumigation. It is only necessary to observe, for the sake of those who may not be versant in chemical pursuits, that the ingredients ought to be pure, and neither metal vessels nor rods should be used. Any kind of metal getting among the ingredients would cause the vapour to be very noxious instead of salutary. The fumes that rise should be white; if they are of a red colour, there is reason to suspect the purity of the ingredients.

The importance of this discovery need not be insisted on: it is equally applicable to every species of putrid contagion, even to the plague itself. It should therefore be used in all hospitals and parish workhouses; and should be constantly referred to by the proprietors of all large works, on the first appearance of infectious disease among the people employed in them:—indeed, it should be employed even as a preventive in all situations where a number of people, from the nature of their business, are obliged to be crowded together, or where, from local circumstances, there are reasons for suspecting that the purity of the air is injured by noxious exhalations or other causes. If there be any circumstances in which its utility may be called in question, it can only be in cases of inflammatory diseases: for, in such, super-oxygenation has been found hurtful.