CHAPTER III.

OCCASIONAL INGREDIENTS IN THE AIR.

AT the extreme end of the valley upon which we are supposed to be looking lies a flat, marshy district, over which in the dewy evenings we may often see suspended a dense cloud of vapour. The whole area of this district is not above three or four square miles, yet its inhabitants are more frequently in ill health, and the annual proportional mortality is greater there than in any other portion of the plain beneath us. Were we to question them, they

would inform us that when in the hot weather of autumn a current of air blew across the marsh, they might certainly expect attacks of ague to ensue. Their very countenances betray their ill-health, and the long and sallow faces of some are so peculiar, that we may well exclaim, What can be the cause of this unhealthiness? The inhabitants say it is the marsh air. When the

same sort of district and effects occur in Italy, the inhabitants attribute it to the malaria, or bad air.

No doubt they are correct.

The air of such

districts contains something in addition to those ingredients which in the last chapter we found to constitute the composition of the atmosphere generally. Nitrogen, oxygen, carbonic acid, carburetted hydrogen, and ammonia, although representing the ordinary ingredients of the atmosphere, are not, therefore, its only constituents in particular cases. Dr. Prout says, "the atmosphere may be conceived to contain a little of everything that is capable of assuming the gaseous form." We shall learn, however, further on, that there are active chemical processes taking place in the air itself which in a short time remove such "occasional ingredients" from its contents.

What is known upon the chemistry of Malaria at present is but unsatisfactory. When vegetable matter is left to rot, with a limited supply of water, and at an elevated temperature, it begins to give rise to certain products of its decomposition which escape into the air, and constitute what is called malaria. Chemistry is in great ignorance upon the nature of these products; but the most curious facts exist, by which, although we can neither determine their

nature nor analyse their constitution, we are yet able positively to affix certain general characters to them. The poison infused into the air appears to be ponderous; this is shown by the fact that it accumulates near the earth, since it is safer to sleep on the top of a house than at the bottom; persons occupying the lower stories having been attacked with ague, while those on the upper have escaped the complaint. It does not appear to be altogether gaseous, for the Italians are in the habit of wearing gauze veils as an efficient protection from it, the infiltered air being thus divested, as they state, of its injurious powers. It is invisible, inodorous, and gives no indication of its presence by any chemical quality whatsoever. A variety of conjectures have been made upon its nature, and some have even supposed that it consisted of minute animalcules. Probably one of the most happy of the explanations given is that which refers it to the existence in the atmosphere of certain minute organic particles buoyant with every wind, coming into existence as a product of the putrefactive process in vegetation, and capable, when inhaled by the lungs and received thus into the circulation, of inducing that peculiar form of disease by which its effects are characterised. But after all it must be confessed, the explanation itself wants to be explained.

The subject of Epidemic Disorders-that is, of disorders affecting at one time large numbers of persons-of all kinds, is equally enveloped in obscurity. A few facts are known, but these are of a sadly insufficient character. Among these is the important and interesting modern discovery, that some diseases, originally local, if they acquire sufficient intensity in the spot where they originate, may proceed, and, gathering additional strength in their progress, eventually become true epidemics-diseases of the people. "Like living things," observes the Registrar-General, “epidemics do not cease with the circumstances in which they are produced; they wander to other places, and descend to remoter times." Thus the accumulating filth of a wretched metropolitan alley may be the hot-bed of a disease not confined to the miserable locality, but extending to the broader squares of the wealthy, to the palace doors, and perhaps inner chambers, of the great and noble, and perhaps descending to posterity. As malaria appears to be an atmospheric impurity resulting from vegetable decomposition, so infectious and epidemic disorders would seem in most cases to arise from the putrefaction chiefly of animal substances, or in some instances from that of both animal and vegetable materials.

PROPAGATION OF EPIDEMICS.

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Reasons exist, to which it is not necessary here to refer, for believing that these disorders of large masses of people are produced by some peculiar organic poison, not gaseous, nor vaporous. It is at least very certain that such organic particles as are detrimental to the health, float in the atmosphere of every great city, and may often be perceived by the senses in the offensive air of the habitations of its poor and dirty inhabitants. Such particles cannot exist in the air without undergoing chemical change, and it is possible that by their existence in this state of change, they may set in motion a series of events which terminates in the appearance of the diseases we are alluding to. A simple experiment will prove the truth of the assertion, that an atmosphere of organic matters is undoubtedly mixed with our air. It is a property of strong sulphuric acid to char or blacken most substances of this kind; now, if a saucer, partly filled with this acid, is exposed for a little period to the air, its colour will alter, and ultimately deepen almost to black, in consequence of a large portion of organic matter having fallen into it from the air, and undergone the charring process; and this will take place even in the open air of the country.

As we look down upon yon distant village, it is not necessary to call in the aid of che