(From contingo, to meet or touch each other; vel infectio, from inficio, to infect). Contagion, or infection.

It has been lately attempted to distinguish these two words, though not with a happy discrimation. We should approach more nearly to common language, if we employed the adjective "infectious"to diseases communicated by contact; for we infect a lancet, and we catch a fever by contagion. In the present state of our knowledge of the subject we must perhaps employ these terms as synonymous, though we shall, in general, follow the distinction now suggested.

Contagion, then, exists in the atmosphere; and we know distinctly but of one kind, viz. marsh miasmata, which probably consists of inflammable air. All moist earth seems to be also injurious; and we now know that moistened earth absorbs the oxygen, and leaves, of course, the azote. Putrefying vegetable and animal substances have also been accused, particularly of producing the yellow fever of America; and there is no doubt of their being highly pernicious, though from what kind of exhalation we cannot say: mineral exhalations seem never to have produced fever.. Another kind of contagion is that which produces the epidemic catarrhs; others which occasion the plague, the dysentery, the cynanche malignata scarlatina. Egyptian ophthalmia, etc. but these we need not anxiously enumerate, as their nature is obscure; and the security, if it can be obtained, rests only on the general principles of avoiding cold, damp, night air, and whatever checks perspiration or debilitates the nervous system. The fevers which may arise from moistened earth cannot be avoided; but those from marsh miasmata we may escape, by choosing a situation where the prevailing winds do not blow from neighbouring marshes; or, at least, where the winds at the period the marshes emerge from under the water, the only time of considerable danger, are not frequent from that quarter.

Many of the diseases above mentioned are "infectious"also; and though it has been lately the fashion to deny contagion or infection, yet numerous are the victims that have been sacrificed to this pernicious system. The plague may still be pronounced eminently infectious. The small pox, the measles, the yellow fever, the ulcerated throat, the scarlatina, catarrhus epi-demicus, and the hooping cough, are probably so, in a decreasing ratio, according to the order. Others, less infectious, require somewhat of more immediate contact. The dysentery, perhaps, connects both classes: then follow the itch, the sibbens, and the venereal disease; perhaps the yaws. Other complaints supposed to be infectious, are apparently so from their being the offspring of contagion only.

We have omitted in this enumeration the jail and hospital fever, as of doubtful origin, partaking both of infection and contagion. The miasmata are undoubtedly diffused through hospitals, jails, and transports; but the diseases usually received from the human body, or the recent effluvia, give activity to the contagion before introduced. Thus we find a person, apparently without fever, coming from an infected hospital or jail will convey the disease, while he himself, having constantly imbibed the poison, is habituated to it; and the infec-tion requires an exciting cause, or the effluvia in a more active state.

Contagious or infectious matter acquires peculiar virulence from confinement, especially with woollens or cotton; and in this way infection is diffused from distant countries. The last plague which infested the town in which we now write arose from a traveller remarking to his companion that, in a former journey, he had the plague in the room where they sat. "In that corner," said he, "was a cupboard, where the bandages were kept: it was now plastered; but they are probably there still." He took the poker, broke down the plastering, and found them. The disease was soon disseminated and extensively fatal.

People are very variously susceptible of infection.

The slightest breath will sometimes induce the disease, while others will daily breathe the poisonous atmosphere without injury. We remember a young lady having a violent fever in a boarding school. Each scholar constantly passed the door of her room, and no one caught the disease. When recovered, she was removed, with fresh clothes, to a lodging, where there was no communication with the persons of the house; but two young women, of the latter, were soon affected with a similar fever, and narrowly escaped. We mention this fact, not to alarm, but to induce the extremest caution, particularly in those who have previously lived in perfectly wholesome air.

The period at which the disease takes place subsequent to infection, is different in different circumstances. Should a case of fever occur within a few hours after a person has been exposed to it, the fever will assume the contagious type. Generally speaking, however, most febrile infections appear active from about ten to fourteen days. The period is scarcely ever less than seven,or more than twenty-one days. Infection is, indeed, more often taken than is supposed. A slight shiver is followed by perspiration, by a diarrhoea, or some other spontaneous discharges, and nothing more is heard of the fever. But its nature is shown by prostration of strength, want of appetite, &c; which continue, at least, during the first septenary period, if not to the fourteenth day. It is generally received with the air inbreathing; but its effect is felt at the back of the fauces only, and from thence in the stomach.

During the progress of contagion, and while exposed to infection, cold chills, indigestion, and the depressing passions, should be as much as possible avoided. The sick room should be kept well ventilated, and the discharges immediately removed: it should be also frequently exchanged for free open air; the mouth and throat gargled with oxymel, a decoction of bark, rendered pungent with spirit of vitriol or port wine. We had almost said brandy, which we once saw recommended in a very respectable scientific journal, the Critical Review, and have since used with some success. But that author was violently reprehended by a gentleman, "scrupulous,"perhaps "over much,"as encouraging dram drinking. We may, at least, pace tanti viri, recommend it to those who have no propensity to that pernicious custom. A decoction of bark, taken two or three times a day, with a few glasses of port wine, may be useful, when infection is around; and an attention to the state of the bowels is essentially necessary. But nothing will effectually preserve without air - free open air.

Fevers, however, caught by recent infection are mild, compared with those which arise from contagion long pent up, styled fomites; and, in our experience, very few such fevers have been fatal. There are other means proposed of guarding against infection. To persons exposed to it, camphor worn in a bag on the stomach has been recommended. We know not its efficacy; but recollect a late recorder of London (serjeant Glyn) mentioning his having worn it in the earlier period of his appointment, but afterwards disusing it. He added, that he thought himself cooler in court since he had rejected it. Another mode of avoiding infection is, fumigations with vinegar and the mineral acids in the form of air. Vinegar has been frequently employed: 3Q 2 but the discovery of the power of the mineral acids is contested. The late Dr. Johnson, of Kidderminster, evidently first proposed this method; but from his pub-lications there appears to be no evidence of his having employed the acid gases. It seems, however, from his manuscripts, that he had done so; and, indeed, the ease with which it could be tried is a strong evidence that he had really used it. Dr. C. Smyth, who received a parliamentary reward for this supposed discovery, seems to rest his claim, in a great measure, on his having substituted the nitrous gas, since the muriatic is so offensive to the organs of respiration; but certainly, previous to Dr. Smyth's application to parliament M. de Morveau had recommended similar trials. We mean not to decide hastily or rashly; but, though the casual notice of this plan by Dr. Johnson is certainly anterior to the hints of any other author, yet to our recollection, M. de Mor -veau preceded Dr. Smyth. Our countryman may not have borrowed the hint; but, in the present state of chemical science, it lay too much on the level for any one to plume himself greatly on the discovery. The method of preparing each gas is extremely simple; and consists only of adding the sulphuric acid to nitre, or sea salt, deprived of their water of crystallization.

We have omitted mentioning vinegar, as it connects the former with some other modes of destroying infection. The volatility of the acetous acid adapts it for this purpose without any preparation; and it is probably highly useful, though perhaps less efficacious than the gases of the mineral acids. It is used in another form, viz. in a highly concentrated state, with aromatic oils. The first idea was taken from a preservative, said to be employed by four thieves, who plundered the victims of the plague with impunity, secured by the aromatic vinegar. It is prepared with great elegance and efficacy by Mr. Henry, of Manchester, and sold under his name.

A more recent plan of this kind is the disinfecting bottle of M. de Morveau. The materials are preserved in a bottle made of strong glass, secured in a wooden case, and the stopper kept down by a screw. In this glass about eleven French drachms of finely powdered black oxide of manganese are put; and to these are added about three ounces by weight, of pure nitric acid of the specific gravity of 1.40, with an equal bulk of muriatic acid, of the specific gravity of 1.134. About two thirds of the bottle must be empty: it is covered with a plate of glass, which is raised by a single turn of the screw. This plate must be ground and polished, so as to close the bottle accurately, and every particle of dust must be carefully wiped off. When used, it must not be held near the nose, as the pure oxygen gives pain when respired; but in a few minutes it will fill a large loom with the gas. These materials, if the bottle is opened daily, will last six months. The price, when properly filled, is, in Paris, twenty one francs for each. Annates de Chimie, vol. lii. p. 347.

M. Dumotiez prepares bottles of a smaller size, with less proportions of the same materials. The chief improvement is, that a small aperture is made by a turn of the screw. They are cheaper, and adapted for apartments of a common size.

When infection or contagion has taken place, an emetic is the first necessary step; and this should be followed by active purging. After these medicines, a warm cordial diaphoretic, and sometimes a blister, is necessary. If, however, any one medicine is peculiarly and essentially requisite, it is the cathartic; but the emetic must be premised. The reasons for this plan will be afterwards explained.

Mineral exhalations have been sometimes supposed to be causes of fever; but we have not found any well authenticated instances of such effects, though Dr. Webster, in his History of Epidemics, has endeavoured with great earnestness to connect epidemics with volcanic explosions, and other physical phenomena. Those who -work in mines of lead or quicksilver are, however, subject to nervous colics and palsies.