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Composition of the Atmosphere.

The pollution of a river, in its progress from its source to the sea, is well shown by the following results of analyses of the water of the Camac, which I made some years ago for certain legal purposes. The river rises in the Dublin mountains, and, flowing for about a dozen miles, commingles its muddy waters with those of the Liffey shortly after the latter enters Dublin.

COMPOSITION OF THE CAMAC RIVER AT DIFFERENT POINTS:

During the outbreak of cholera in 1865 I found the pump water in Arklow, Mallow, and other towns ravaged by cholera in a very impure state, and I traced the spread of the disease, in many instances, to the use of foul water. At that time, however, the methods of water analysis were not so perfect as they are now,

CHAPTER X.

NORMAL AND ABNORMAL AIR.

The atmosphere by which our globe is invested extends to a height of forty-five miles-or, according to some observers, seventy miles-from the level of the ocean. A cubic foot of dry air, free from carbonic acid, weighs 537 grains (nearly 1 ounces), when the thermometer stands at thirty-two degrees Fahrenheit and the barometer at thirty inches. A column of air, extending from the ground to the extreme limits of the atmosphere, balances a column of mercury thirty inches in height, and one of water thirty-four feet in height; and it presses upon the surface of bodies with a force equivalent to fifteen pounds weight upon each square inch. The air decreases in specific gravity as it recedes from the earth. At the summit of Mont Blanc half the weight of the atmosphere is lost, the mercury in the barometer sinking to fifteen inches. The atmosphere consists of several gases and vapours in a state of mechanical admixture, not of chemical union.

AVERAGE CENTESIMAL COMPOSITION OF THE ATMOSPHERE.

In addition to these substances, many others frequently occur, especially in the air of towns; for example, sulphuric, sulphurous, muriatic, and nitrous acids, chlorine, phosphuretted hydrogen, common salt, and various earthy and saline matters, and metallic

vapours.

Oxygen gas is the most important constituent of the atmosphere; for most of the functions discharged by that fluid are, in reality, performed by oxygen. The respiration of animals and the ordinary processes of combustion are solely maintained through its agency; and many of the great changes continuously passing over the face of Nature are the result of the action of this potent element. Oxygen is about one-tenth heavier than atmospheric air. It is destitute of colour, odour, and flavour; 100 gallons of water dissolve between three and four gallons of it. No animal can exist in air which has been deprived of its oxygen, and fishes speedily expire when placed in water free from this vital element. Oxygen destroys the organic matters found in the air and in soils, by uniting with their carbon and hydrogen, and converting those elements into the carbonic acid and water. It also destroys vegetable colours-a property of which the linen and cotton manufacturers largely avail themselves, exposing their brown or yellow fabrics to the bleaching action of the air.

For a long time it was supposed that the relative proportions of oxygen and nitrogen in the atmosphere were the same under all circumstances; but recent and more accurate investigations into the composition of the air have shown that it is not constant. The variations, however, are within very small limits, and are produced almost solely by the processes of the combustion of fuel and the respiration of animals.

Ozone is oxygen gas in some peculiar condition, the precise nature of which is not thoroughly understood. It possesses a disagreeable odour, which some compare to that of diluted chlorine, others to that of phosphorus. We notice this odour during a thunderstorm, when phosphorus is exposed to the air, or when an electrifying machine is developing electricity; for, under those circumstances, some of the oxygen of the air is converted into ozone. It is a powerful bleacher, and readily decomposes several chemical compounds. It is probable that some of the effects of atmospheric air upon animals and vegetables, and their products, are produced by ozone. This remarkable body is formed by passing electric sparks through the air, or by slowly oxidising moist phosphorus.

Epidemics of influenza have been attributed to an excess of ozone in the air; and its absence from the atmosphere is supposed to favour the spread of if not to originate, certain zymotic diseases. During the outbreak of relapsing fever in London in 1870, Dr. Ross, Medical Officer of Health for St. Giles's District, noticed that the fever decreased inversely as the ozone in the atmosphere increased. When the mean amount of ozone was so low as 0.4 at

96

Antozone-Nitrogen.

Greenwich, the cases of the disease amounted to 3.4 each day; when the ozone rose to 1, the attacks decreased to 3 per diem. The days on which the ozone stood at 2, the attacks were 1; at 3, the cases of disease were 0.9 daily; and when the ozone increased to 5, the attacks sank to 0.3 daily. These facts certainly do not appear to be mere coincidences. They show a close relation between the amount of atmospheric ozone and the intensity of a zymotic disease. They do not, of course, prove-that which has been so often alleged that deficiency of ozone in the air is the direct cause of disease; but they certainly go far to prove that ozone is capable of oxidizing the morbific agents which float in the air. It is to be regretted that Dr. Ross did not determine the amount of ozone in the atmosphere of his district. St. Giles is some distance from the Observatory at Greenwich. At the same time, it must be admitted that the amount of ozone in the air at Greenwich would be about the quantity which would exist in the atmosphere of all parts of the metropolis, provided it were not used up in oxidizing or ozonizing the organic matters and sulphuretted hydrogen gas which occur in sensible quantities in "London smoke." It would be desirable that every medical officer of health should provide himself with the most modern and useful form of ozonimeter. The mode of using such an instrument is very simple; and if it were constantly employed I have no doubt but that valuable medical results would follow.

A piece of paper or linen, soaked in a solution of starch and iodide of potassium, is a test for the presence of ozone, acquiring a blue or lavender tint on exposure to air containing a very minute trace of ozone. This curious substance can rarely be detected in the air of over-crowded rooms, or other places where the air is very impure. It is most abundant in the air above the ocean, and is seldom, if ever, absent from the atmosphere of the open country, because it is used up in destroying the organic matters which are so abundant in such situations. Ozone is abundant during snowstorms; and it is supposed that the bleaching power possessed by newly-fallen snow is due to its presence.

Antozone is a term applied to another form of It is oxygen. supposed to be produced when ozone is formed, the oxygen being split up, so to speak, into these two curious bodies. Antozone has a disgusting odour, but a less pungent one than ozone. It soon passes into ordinary oxygen. It has the curious property of forming fogs or clouds with water; perhaps it is the common cause of these phenomena. It must, however, be admitted that great uncertainty still prevails relative to the nature of antozone, and there may be no such substance in existence.

Nitrogen is colourless, tasteless, and odourless. It is a little lighter than atmospheric air. One hundred parts of water dissolve only two parts of this gas. Nitrogen cannot support combustion or respiration; but by diluting the atmospheric oxygen, it renders that gas less stimulating to animals.

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Watery Vapour-Carbonic Acid.

Watery Vapour varies considerably in amount, being sometimes so low as 0.5 per cent., at other times so high as 4 per cent. The average amount is about 14 per cent. The warmer air is, the greater is its power of maintaining water in a vaporous state.

Carbonic Acid Gas is a compound of the elements carbon and oxygen; its specific gravity is 1-5224, water being 1. It has no colour, but posseses a slightly pungent odour, and its solution in water is feebly acid. It is fatal to animal life, even when largely mixed with atmospheric air; and a lighted taper is extinguished in air containing only a small percentage of this gas. It is extremely soluble in water, in which fluid it is invariably present, and often in large amounts. It is this gas which is driven off from limestone when that substance is ignited in kilns; and its presence imparts to champagne, bottled malt liquors, and mineral waters much of the agreeable flavour which those beverages possess.

The amount of carbonic acid gas in the air varies from three to six parts in 10,000 parts of air; the average is 04 per cent. It is sparingly present after rain, and accumulates during seasons of drought. It is more abundant in summer than in winter, at night than during the day, and over land than above water.

Carbonic acid gas furnishes the largest portion of the carbon used as food by plants; and the nitrogen of vegetables is to a great extent derived from ammonia. It is evident, then, that these gases are normal ingredients of the atmosphere; and it is not likely that, unless when present in excessive proportions, they exercise an injurious influence upon animal life.

Carbonic acid, though food for plants, is poisonous to animals, if taken into the lungs in large quantities. When limestone is strongly heated, nearly half of it is dissipated in the form of carbonic acid. Many poor, homeless creatures have lain down close to the limekiln fire, to enjoy its warmth, but, succumbing to the narcotic influence of the carbonic gas, have fallen into a sleep from which they never awoke on this earth. The carbonic acid which accumulates in brewers' vats, in the holds of corn-laden vessels, and in some other situations has often caused the death of persons who incautiously descended into it.

senses.

Dr. Smith states that an increase of atmospheric carbonic acid from 0.04 to 0.07 per cent., is in general recognizable by the An increase of even 0.02 per cent. is not pleasant to us, when caused simply by want of ventilation; but if the increase be associated with an evolution of the gases of putrefaction -which is often the case-the deviation from the healthy atmospheric standard is the more dangerous to health. Indeed, the results of the experiments of Dr. Smith and those of Dr. Hammond, of the United States, show that the ill effects experienced by those who breathe air vitiated by respiration are chiefly due to the organic matter; for if the carbonic acid be removed by chemical means, the respiration of the air continues to produce an injurious