Metropolis, where, on cold still days, it gorges itself on its arrival with compounds of soot, sulphurous and carbonic acids, and other noxious abominations, and soon acquires a sufficiently substantial percentage of poisonous matter to render it an even more perilous enemy to human life than is yet comprehended by the general public. Thus, although it is obvious that every addition to the quantity of coal consumed in London has added, and continues to add, to the percentage of poisonous matter with which the air is daily infected, yet, strange to say, the abolition of the coal duty was advocated and carried out three or four years ago, with the avowed desire of stimulating an increase of manufactures, and an increase in the consumption of coal, in the very midst of our dense population.

The evil effects on the atmosphere of London which result from the already excessive consumption can only be adequately realised by a careful consideration of the manner in which all combustion is brought about. When burning coals in the grate, we are unconsciously promoting a chemical process which consists in effecting the combination of oxygen derived from the air (or "draught ") with the carbon contained in coal in the proportion of 2 pounds of oxygen gas to one pound of carbon; the result of which is that for every pound of carbon burnt in the grate we send up the chimney and into the atmosphere above it no less than 3 pounds of the compound gas, that is to say, of carbonic acid (CO2), which is of a nature so poisonous to the lungs and respiratory organs that it becomes injurious to human life when the quantity contained in the atmosphere rises to the proportion of 5 volumes of carbonic acid to 1,000 volumes of air. Coal contains also about one per cent. of sulphur, and, in addition to carbon, a certain proportion of hydrogen; and perfect combustion is obtained when one part by weight of carbon combines with 2 parts of oxygen to form carbonic acid, and one part by weight of the hydrogen unites with 8 parts of oxygen to form water, or steam, or with 8 of sulphur and 24 of oxygen to form sulphurous acid.

To accomplish this combination the oxygen has all to be taken from man's own vital provision and supplied to the burning coal by means of a draught of air passing over or through it. The air is composed of oxygen in combination with a diluent in the form of nitrogen, in the proportion of 23 parts by weight of oxygen to 77 of nitrogen, and the 23 pounds of oxygen which are necessary for the combustion of one pound of carbon are contained in about 160 cubic feet of air; but it is found in practice that at least double that quantity of air must pass through the grate in order to effect the

combustion of each pound of burning carbon. A daily consumption of 40,000 tons of coal in London implies therefore the daily combustion of no less than 28,672 million cubic feet of air, and the daily production of about 330 million pounds, or say 147,000 tons weight, of poisonous carbonic acid !

These figures conclusively show that, although it is customary to regard the act of combustion as one in which the principal fuel burnt is that which we have perceptibly before our eyes in the form of coal, or coke, &c., the truth is that combustion is maintained by relatively small quantities of carbon and hydrogen derived from the visible fuel. These are combined with a far larger quantity of diluted oxygen supplied to the fire by the air, or draught, which, only because it is in a form invisible to our eyes and apparently costs nothing, is often too little considered; and perhaps if we were accustomed to speak of combustion as a process in which air is the principal, and coal the auxiliary element-if we were to say that we burn air in our grates instead of saying that we burn wood, or coal— such a definition would lead to a clearer comprehension of the vast amount of contaminated air that is contributed to the atmosphere of London by the enormous consumption of coal, which the large population in this metropolis renders necessary.

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The products of combustion of coal comprise also sulphurous acid and smoke, or soot-the colouring ingredients of fog-and other compounds which are exceedingly deleterious and unpleasant, but carbonic acid is the really dangerous ingredient of fogs, owing to its much greater abundance. We are so much habituated to trust to Nature, that is to say, to the wind, for the ventilation of London that it may seem hardly credible, although it is a well authenticated fact, that if that natural ventilation should, under any circumstances, be suspended pending the continuance of a bad fog, that is to say, there should be a "dead calm," the whole population might be poisoned by the carbonic acid with which the air would in that space of time be saturated. Only those whose occupations oblige them to be much exposed to a dense London fog can have any idea of how nearly that dangerous condition of the atmosphere is sometimes approached, both as respects absence of wind and the extreme difficulty of breathing caused by the excess of carbonic acid, especially in low-lying districts and in streets where there is no traffic to keep the air in motion.

In an interesting paper on the atmosphere Sir Henry Roscoe has given the results arrived at by himself and many other observers with regard to the proportion of carbonic acid which renders air injurious

to health. "From very numerous observations" (he says) "made by Saussure, Brunner, Lewy, and others, it appears that air in the open country contains quantities of carbonic acid varying from three to ten volumes in 10,000 volumes of air. As an average number it has been found that four volumes in 10,000 represent the usual composition of air as regards carbonic acid. . . . Whilst forming the staple nutriment of the vegetable world, carbonic acid, when present in certain quantities, acts as a violent poison on the higher orders of animal life; nor is the limit at which this gas begins to be hurtful to the animal very far removed from the quantity which at present exists in the atmosphere: for we find that Leblanc and Péclet assign a limit of fifty in 10,000, whilst Reed and Arnott give a much lower limit to the non-injurious effect of this gas. . . . We know that there are a great number of causes continually at work, some of which tend to increase the amount of atmospheric carbonic acid, whilst there are others which tend to effect its diminution, so that a continual circulation of atmospheric carbon takes place. The animal gives off the waste portion of its body, mainly as carbonic acid, and thus deteriorates the atmosphere, which would soon become unfit for his further use if the vegetable world did not absorb the poisonous gas, at once retaining the carbon in the solid form and exhaling the oxygen, wherewith the higher organism again removes his spent materials. . . . The amount of carbonic acid present in dwellingrooms, &c., has been made the subject of experiment by Leblanc, Pettenkofer, Roscoe ('Chem. Soc. Quart. Journ.' x.), and Smith, and their results show that, whereas in well-ventilated rooms the amount should never exceed eight volumes in 10,000, in badly-ventilated rooms it rises to ten and twenty volumes, and sometimes even as high as seventy volumes in 10,000."

Now, we have already seen that the quantity of carbonic acid that is produced by the daily combustion in London of 40,000 tons of coal amounts in weight to 147,000 tons, which is equal to a volume of 2,838 million cubic feet; and, in the ratio assigned by Péclet and Leblanc of fifty in 10,000, that quantity of carbonic acid would, in a perfectly still atmosphere, render injurious to health no less than 567,600 million cubic feet of air. Such a volume of vitiated air represents a column which, if extending twelve miles in all directions around Charing Cross, would reach to a height of forty-five feet; while a five-days' fog, accompanied by an almost absolute cessation of wind, such as London has sometimes experienced, if ever it should entirely paralyse ventilation and hinder the removal of the products of combustion, would contaminate a similar column to the extent of 250

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volumes of carbonic acid to 10,000 of air, which is a degree of impurity that would much more than suffice to suffocate and destroy the whole population and all animal life throughout the Metropolis.

Nor should the circumstance be lost sight of that the consumption of coal in London and its suburbs is rapidly increasing. During the last fifteen years it has increased 50 per cent., and a similar rate of progress would raise the daily consumption to 60,000 tons in fifteen years' time, and to 90,000 tons thirty years hence; so that we have to look forward to a period not very remote, when the quantity of carbonic acid that will have to be dealt with will be double what it now is. It may, therefore, be confidently asserted that the time. has arrived when the inhabitants of the Metropolis ought to regard the subject as one that involves imminent danger in the near future to every individual, and one that demands immediate and serious attention from the London County Council.

Such partial remedies as have usually been advocated would plainly be very insufficient. Even the total suppression of smoke in domestic chimneys, which would diminish its filthiness and, to a certain extent, improve the colour of fog by lessening the number of sooty particles that it contains, would not affect the innumerable smoky gas flames which burn in the open air, and neither would it decrease the amount of sulphurous, nor of carbonic acid, which are the worst enemies to the respiratory organs. The drainage of the marshes where much of the fog originates would be an equally incomplete remedy, and it would, in fact, be useless to expect thorough relief from the infliction by any less efficacious means than those by which Nature herself ordinarily clears the atmosphere. We never have a fog when there is a high wind, and indeed a gentle breeze is quite sufficient to blow it away; while, in these days of improved ventilation, it might not be an impossible task to rid ourselves of a dense fog by an artificial imitation of Nature's gentle breeze, than which there is no more wholesome purifier of the atmosphere. Whether it would be possible to effect this out-door ventilation on a complete scale is practically a question of cost, and consequently a fit subject for calculation and discussion and the first step towards a solution is a determination of the quantity of air that would have to be set in movement, and of the mechanism that would be necessary in order to render such artificial-wind-ventilation successful.

It must not be assumed that ordinary fogs cover at any one moment the whole of the area comprised within the twelve-mile radius, for that is very rarely, if ever, the case. Howard, who was the first to classify clouds, and published a work in three volumes on the

"Climate of London," containing his daily journal of the weather during many years, describes several very dense fogs. Of one of these he says: "No further to the northward than the back of Euston Square the weather was clear and bright, and southward of London the fog extended as far as Clapham ;" of another he writes: "At one o'clock the fog in the City was as dense as we ever recollect to have known it. Lamps and candles were lighted in all the shops and offices, and the carriages in the streets dared not exceed a foot-pace. But at the same time, five miles from town, the atmosphere was clear and unclouded, with a brilliant sun." Again: "The fog of Wednesday has seldom been exceeded in opacity in the Metropolis and its neighbourhood. It began to thicken very much at about half-past twelve, from which time until near 2 P.M. the effect was most distressing, making the eyes smart, and almost suffocating those who were in the street, and particularly asthmatic persons. It cleared off about 2.15 P.M., but returned with all its intensity in the evening."

These are very much like the dense fogs which we have all experienced. Owing to the almost complete absence of wind at such times, the products of combustion, which are slightly heavier than the moist, foggy atmosphere, are unable to rise, and they sink from the chimney-pots to the level of the streets, exposing the passers-by to the danger of asphyxiation. But the comparatively limited area over which fogs extend encourages the belief that the volume of air which would have to be set in motion might be adequately dealt with by creating such upward currents of ventilation as would be produced by a number of air-propellers, or ventilating fans, placed in succession along certain routes. The characteristic stillness of the atmosphere during fogs, and the tendency of these to stagnate, would by this means be neutralised, for innumerable currents of wind would be artificially created, breaking up the fog and drawing before them the noxious products of combustion, as they issued from the chimneystacks.

Without entering into unnecessary detail, the following figures may be taken as sufficiently indicating the means by which the ventilation of the streets could be effected, and its approximate cost, on the assumption that 40,000 tons of coal are burnt every twenty-four hours, and that the amount of fog to be driven daily through the fans should be at the rate of 300 cubic feet for every pound of coal that is burnt. If air-propellers, two feet in diameter, are used for this purpose, each of these would deliver and drive upwards a column of 160,000 cubic feet of air, or fog, per hour; and, if placed at a distance of fifty yards apart, there would be thirty-five propellers per mile, setting in