and legislator will always regard as a serious evil, and endeavour, as far as possible, to prevent-in warm climates, more especially and anxiously.

Having thus discussed at some length, the several acknowledged and probable sources of Malaria, we go on to treat of the circumstances which favour its production and propagation. We find the intensity or malignancy of the diseases occasioned by miasmata, very regularly proportioned to the degree and permanence of the heat to which the materials evolving them, and the systems to which they are to be applied, are subjected. Thus, in hot countries we have the plague, (if this be a Malaria disease,) yellow fever, hepatitis, bilious remittents and malignant intermittents. As we proceed northwardly, we meet the slower but scarcely less fatal typhus, remittents and ordinary intermittents, chronic hepatitis, &c. There is every reason to believe that heat not only aids essentially in the developement of the noxious agent, but tends also to effect in the human body, a predisposition to the morbid influence exerted by this agent. Heat stimulates, and by its long continuance relaxes the surface, and brings on a state of debility, languor and prostration, which much favours the invasion of any malady.

To give efficacy to the force of heat in the decomposition of vegetable remains, the presence of moisture is absolutely necessary. In a state of perfect dryness, the leaves of plants and flowers may be preserved forever. But this is not all. The operation of moisture is extended beyond its mere influence in promoting the formation of Malaria; its presence would seem further, positively essential to the effectual application of the poison to the living surfaces. Fogs have, in all ages and among all nations, by the vulgar, by poets, and by philosophers, been regarded as either in themselves insalubrious, or as the medium of conveying or concentrating the various agents of destruction diffused in the air around us. But the fogs of northern and elevated regions, and of winter, are not injurious, unless in the obvious mode of lowering the temperature of the body, against which we can easily and effectually protect ourselves. It is not then of themselves, but by means of the poisonous atoms which exist in intimate union with the vapour, perhaps, in a state of actual chemical solution, that they give rise to the consequences attributed to them. Situations particularly liable to immersion in fogs are, in a corresponding degree, subject to Malaria diseases. The immediate banks of our water-courses, and the lower portions of our sea-port towns, immediately adjoining the wharves and docks, may be taken as examples. The times of day at which fogs and dews rise into the atmosphere, and con

dense and are precipitated from it, have always been regarded as periods the most dangerous to those exposed. The hours immediately preceding sunrise, and those which follow sunset, are avoided with care by the natives of miasmatic regions. At Rome, the streets are silent and solitary during the early evening in summer and autumn, but are again filled with a gay and lively throng as the night progresses, and the dews are supposed to have fallen. Night is, however, a time of peril throughout; and, perhaps, with us who suffer nearly as much from the heat of our nights as of our days, the condensation of atmospheric moisture may be said to be almost equal through all the hours of darkness. The least risk is imagined to exist at mid-day, when the air is rarified, and the aqueous exhalations with which the Malaria is supposed to be combined, are carried up into the loftier regions of the atmosphere by the vehement heat of the noontide sun. The knowledge of these facts we have from experience and observation, and every day's attention confirms their correctness.

We repeat, that it is not by or of itself, that moisture produces the effects denoted. It is after the subsidence of the sacred river that Egypt suffers. Our author mentions many examples of the increase of Malaria upon the draining of a neighbouring pond, or lake, or canal. An impressive instance is given by Rigaud De Lisle. "At the time of the erection of the bridge of Felice, in order to unite all the waters of the river, Sextus V: was obliged to divert a branch of the Tiber, which passed below the hills of Magliano, leaving to time the task of filling the old bed. Half of the population perished; one single convent of nuns contained 69 sisters, including novices, of whom 63 died in two years.'

It is well known that the lowest grounds, if inundated by very abundant summer rains, will often escape or suffer less from their usual endemics, while in dryer years, the swamps being evaporated to comparative shallowness, their rank and fermenting margins are exposed to the solar heat, and give out in infinite abundance and intensity, the noxious effluvia. A very regular correspondence has been observed to exist between the quantity of rain which falls within the limits of our own city, and the prevalence of our autumnal diseases; but the sickly season does not commence until after both the rains and the heat of the sun have begun to decline. In June and July, for instance, the sun is hottest, and apoplexy and insolation occur in our streets. In July and August, our greatest rains fall. Our fevers are most rife and deadly in September and October.

The influence of different seasons upon the salubrity of miasmatic regions, is analogous to what has been stated. A very dry summer and spring by depriving the decaying vegetable materials of the due amount of moisture necessary to the full and rapid developement of miasmata, diminishes the intensity of the febrific cause. A very rainy season produces a like effect, provided the rains continue throughout the autumnal months, by covering the whole noxious surface, and thus preventing or very much lessening the exposure to heat. Thus, we readily explain the common observation, that in wet autumns the higher grounds, even the sides of lofty hills and mountains, suffer the evils, which, in ordinary years, are confined to the vallies and water-courses. In a similar way we account also for the circumstances attending the immediate fall of rains. If these are large and copious, they combine with and bring to the earth and wash away the atmospheric effluvia, and carry off in the torrents which they occasion, a large portion of the soft loose decaying vegetable surface. Slight showers, on the contrary, falling on the hot soil, are at once evaporated and rise into the air in the form of mists and vapours, bearing on their damp wings dense masses of pestiferous exhalation.

The action of the electric fluid upon these ærial poisons, is far from being well understood. An opinion has been very generally prevalent, from time immemorial, that its agency is highly salutary, whether by any effect upon the miasmata, specific or chemical, or by a beneficial influence exerted directly upon the human constitution. It is by no means improbable, that electricity may act in both these modes; but our notions concerning its protective or remedial power, must be confessed to be as yet entirely conjectural. It must not be forgotten, however, that thunder storms always esteemed so salutary, are attended with great agitations of the air, a violent tumult of the elements. If good result, as it undoubtedly does, from these concussions of the tempest, we must not attribute it exclusively to the beneficial play of the forked lightnings. Strong winds disperse the foul mephitic exhalations, and driving them forward with infinite force and rapidity, diffuse them through space, and thus counteract in a mechanical way their malignant activity, by destroying the concentration upon which depends, as we have before had occasion to observe, their virulence and effect. Calms, by allowing and favouring this concentration, give to all ærial poisons an indefinite augmentation of intensity, as has been noticed from the plague of Athens down to the present time. Hence, we find the sufferers in cities devoted to any form of pestilence, endeavouring with all the energy of despair, to set in motion currents of air

by artificial means; by the gloomy fires kept burning in the desolate streets, by the firing of cannon, and the explosion of large masses of gunpowder.

We are next to notice the influence of more moderate winds in the propagation of Malaria. Clouds are carried to unknown and indefinite distances without dispersion. Fogs and vapours occasionally travel far. Moisture in these forms, and even in its union with air as in ascending and descending dews, and in hygrometric diffusion through the atmosphere, may yet retain its connection with Malaria, and thus convey the latter upon the viewless wings of the wind to regions remote from the source to which it owed its origin. The eastern and northern banks of our rivers, lakes and swamps, are more liable than the opposite sides to autmnal fevers, on account of the general prevalence of westerly and southern breezes; a change of wind at once effecting the more healthy margins. Rush maintained that Malaria might be thus wafted perhaps thirty or forty miles, and our author quotes instances of its having been borne far out to ships at sea, and accounts for the agues which follow in certain parts of England, the setting in of easterly winds, by supposing "that the east winds which reach that country from Holland, may bring the Malaria thence with them, even though they should not convey a fog or a cloud at the same time."

It seems that this east wind has been charged by common prejudice with the possession of some inherent peculiarity, by which it gives rise to intermittents-against which opinion our author vehemently argues. In confirmation of his views as to the innocence of this wind in that respect, we would offer the remark in passing, that there is no breeze more salubrious to us, and the reason is obvious; although charged with moisture, it blows directly from the ocean, and its dampness is free from all miasmatic combination.

With regard to the nature of Malaria, all the hypotheses hitherto proposed, are crude and conjectural. "That the poison of marshes consisted of animalculæ invading the body through the lungs," says Dr. MacCulloch, "sometimes, I presume, through the stomach also, is a speculation which dates as high as Lucretius, Varro, and Columella; which seems to have been renewed in the days of the microscope, by Kircher and some others, and appears naturally enough to have found favour with Linnæus." We must not speak lightly of a theory which numbers such names among its advocates, and which has, in our own day, received support from the learned and ingenious Dr. Cooper, who is, indeed, inclined, if we do not mistake his views, to agree fully with Linnæus, in attributing every form of disease

to the agency of specific animalculæ. We can only say, that all the facts, and observations, and arguments, offered upon the subject, leave the evidence still deficient, and only avail to prove the possible truth of the speculation.

It seems exceedingly reasonable to expect some aid from chemistry in this matter, and all the irrespirable gases known to be produced in any notable abundance by natural processes, have been suspected of constituting the essential ingredient in miasmatic exhalations, and we find among the eminent persons favourable to this notion, Orfila and Volta. But from the very nature of the case, we may consider the question decided in the negative, by the want of positive testimony. Our eudiometers are so delicate, that some regular result would undoubtedly have been obtained by the accurate experimenters who have engaged in the investigation, were this the true path of discovery, but it is now, we believe, universally admitted, that the air of the most pestiferous marsh or jungle, is composed of precisely the same imponderable materials mingled in precisely the same proportions, with that which is to be found in the most favourable and healthy positions.

We are at last driven to the conclusion, that from the various sources of Malaria, a certain subtle something is evolved, which makes, perhaps, no impression upon any of our senses, which will not affect any of our re-agents, whose presence, therefore, can only be known by its influence upon the constitution. Our author seems disposed to look upon it rather as a compound than a simple substance. He thinks that the Malaria of different places, may not be exactly or chemically identical, "since he cannot conceive how mere differences of quantity should be so constant or produce such uniform effects" in the differences observed between the diseases of various climates and districts. We see no difficulty, however, in attributing all these differences to the various conditions of mankind, built up upon peculiar habits as to diet, drinks, clothing, labour, &c. where they are not accounted for by climate or intensity of cause.

The atoms of Malaria would rather seem to resemble in nature, as they do in minuteness, the odoriferous particles thrown off from certain bodies, than those of the air or gases. The particles of contagious virus diffused in the air around those sick of particular diseases, possess also many points of resemblance. We view it as highly probable, that we shall hereafter acquire the means of ascertaining the presence of these peculiar substances, when we shall have arrived at a better and more distinct knowledge of the properties which they possess. Their minuteness we do not think an insurmountable obstacle,