And if we thus account for the variations in the quantity and virulence of diseases in any given country, for noted seasons of epidemic in the countries which I have just named, and for the great prevalence of fevers among ourselves during the last few years, and particularly in the last summer, there is another point of scarcely inferior importance to be taken into the consideration, independently of that which relates to peculiar winds as connected with the propagation of this poison;—and this is, moisture.
I need not repeat that water in some form is necessary to the production of that peculiar vegetable decomposition which is the source of this poison; and so true is this, that even in the tropical regions, the diseases from this cause are nearly unknown in districts of peculiar dryness, as they are in the drier seasons of those countries. Thus, for example, Egypt is free from such fevers, except at the period of the subsidence of the Nile, unless where, as at Damietta, the cultivation of rice is pursued; and the same is true of Mesopotamia very remarkably: and if I dare not extend these illustrations, I must remark that in all these cases, the action of moisture is twofold, inasmuch as it not only accelerates vegetable decomposition, but renders the atmosphere a fitter conductor of this poison.
Taking these two causes of the increase in the quantity and in the action of malaria, we can explain many particulars which relate to its power in producing diseases: and as the knowledge of these is important as far as relates to the main object of this paper, prevention, it becomes necessary to explain them at a little more length.
As to season, the simplest case is that of the intertropical climates; and Africa offers the plainest instance among the [p041] whole. There, the malaria and the fever commence at the moment the rain falls; diminishing as the ground becomes thoroughly wetted, and recommencing as it dries. The explanation of all this ought to be obvious; and the same analogy governs all the hotter climates, as, though less conspicuously, it does our own. Hence we explain, both as to our spring and our autumn, the effects of heat following rain, or the reverse, and the diseases which are consequent on those changes: and thus it is, though more remarkably, in Italy, that a rainy autumn increases the number and severity of fevers; or, if the summer has been unusually dry, that they often do not appear till the commencement of the autumnal, or even the winter rains. And hence, also, even with us, the occurrence of a single rainy day or week, in the midst of the heats, will produce fevers; while the effect of this influence is such, that should there even be an entire rainy summer, and the subsequent one be hot and dry, this will be attended by an unusual production of malaria and disease.
And if I cannot detail all the various modes in which these circumstances may be modified, and how their effects may vary, it will be useful to make one remark on an error as relating to it which is universal among us, and into which even Lind has fallen. The error is, to think that the rain, the moisture, or the cold is itself the cause of the diseases which follow this state of things; while it is obviously a case analogous to that of Africa, if less severe, and the malaria is produced by these circumstances on soils which I formerly pointed out, and which Lind, like every one else, had neglected. But if I must pass over many interesting and useful conclusions to be drawn from these general principles, there is one fact which I must notice, and it is this:—
In spring, the combination of heat and moisture, easily explained, generates, most commonly, intermittents; or the effect of the malaria at this season differs from what it does in autumn: while as the heat advances and the ground dries, this kind of fever ceases to be produced, a new species, or the summer remittent, taking its place when the heat and the moisture of autumn begin to act. But under peculiar seasons of heat and moisture with us, it sometimes occurs, as it has done [p042] within the last years, that the intermittent season runs into the remittent one, or there is no midsummer interval of freedom from disease; while it has also happened, and in some parts of England in this last year, that what would have been intermittent fever in other years has been remittent; or the common fever has occupied the whole summer, continuously, even from March to November, as is the case in the worst regions of southern Europe.
Now, under these exceptions, which I was bound to explain, the commencement of intermittent, or of vernal ague, may be fixed about the middle or end of March, and its termination similarly in May; while that of remittent may be placed in the beginning of August, and its termination with the middle or end of October. How these periods may otherwise be affected by the more or less insalubrious nature of the district or place, will easily be judged of by those who will reflect for themselves on what I dare not explain, lest I should infringe too far on my limits. All else that I can venture on, as to this part of the question in hand, relates to the effects of the different times of the day on the production, propagation, or influence of malaria, and it is one which is of no small importance in a practical view.
Whether the changes as to temperature and moisture which occur within the space of twenty-four hours, affect the production or propagation of malaria, I will not here inquire minutely, from the fear of prolonging this very limited paper; but the general facts, as to its effects, are these: If we commence with the sun on the meridian, there appears, even in the worst climates, very little hazard of fever; while in Italy, it is believed that there is, generally, little or no hazard, except in some peculiarly pestilential places, and under particular kinds of inattention or neglect. Either the malaria is decomposed or destroyed by the heat, or else the air from its dryness ceases to be a conductor; but as evening approaches, its influence becomes powerful and dangerous, being supposed most generally to extend all through the night; while in some parts of that country it is a popular belief that it terminates before midnight, or with the precipitation of the atmospheric moisture. Whether this last opinion is true or not, the general fact explains the popular [p043] belief, and truth, respecting the poisonous effects of dew in the hot climates; the supposed pernicious quality of this depending evidently on the malaria by which its formation is accompanied. And in this case it is probable that the evil arises, not from a fresh or peculiar generation of malaria, but from the mere fact that the moist atmosphere is a better conductor than a dry one.
Not to be unnecessarily minute, we thus also explain the danger of exposure to the morning air in similar situations; the facts, as they relate to the conducting of malaria, being the same, though the meteorological circumstances are somewhat different. Hence, also, we see why the grey mists which hang over wet grounds in the evening in our own climate, are esteemed pernicious; the truth, however, being, that they are perfectly innocent at certain seasons and in certain places—as in the greater part of Scotland, for example, or in those places and at those periods where malaria is not produced. The distinction is valuable, because of the inconvenience of restrictions on this subject, and because to know where the hazard really lies is to reduce those, and also to prevent the infraction of rules by not extending them beyond what is necessary; and thus also by seeing what are the real dangers of what is called night air, we more easily avoid them. Night air is avoided now, under a false philosophy, because it is cold or damp, or for some other vague reason; while the dangers from mere dampness or cold are as nothing compared to those here pointed out; which also occur precisely where they are least feared, namely, in warm summer evenings, after refreshing showers, and so forth. Hence it is that fevers are produced in summer, in rural situations, and especially perhaps amid the most engaging scenery, by evening walks and exposure to what is naturally considered, as it is felt to be, a balmy and refreshing sequel to a hot day. Let this be enjoyed where it can with safety, and as it often may; but such evening walks will not be safe in any of those situations which I need not repeat here; after having detailed them as I have done in the former paper. And lest I should be accused of wishing to excite unnecessary alarm, I consider, on the contrary, that it ought to be diminished by these remarks; because, if we take the whole of [p044] England, there is perhaps not one acre in a hundred thousand where there is danger from night air, or from malaria in any mode; so that to distinguish where that lies, is to have relieved from useless fears all those who may learn to make the distinctions under review.
To pass from what relates to climate and season, and to proceed to the propagation, simply, of malaria, it is almost superfluous to say, that its influence, as to the production of disease, is much regulated by proximity, which implies a state of concentration or accumulation. Hence the danger arising from vicinity; while, as I formerly remarked, where the generating source is small, this becomes necessary to its effect, since dilution may be expected to destroy the power of the poison.