Page images
PDF
EPUB

grm.

0.0290 of Phosphorus when burnt to P,O, gives 16.7 units of heat.

5

[ocr errors][merged small][ocr errors][ocr errors][merged small][merged small][ocr errors][ocr errors][ocr errors][ocr errors][ocr errors][ocr errors][merged small][ocr errors][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small]

§ 327. The atmosphere is that part of the globe which is in a gaseous state. Since every gas is lighter than every liquid and every solid, it follows that the atmosphere forms the external coating of the earth. The attraction of the earth keeps the atmosphere in contact with the earth. Since all gases tend to mix with one another, the atmosphere contains in almost constant proportion all the permanent gases which are in it. Since the atmosphere has weight, and since the density of a gas varies directly as the pressure to which it is exposed, it follows that the lower or more earthward parts of the air are the more dense.

§ 328. Nothing is known with regard to the superior or external surface of the air. There is no conclusive evidence to prove that it has any limit whatever. But the higher we rise the less dense becomes the air, until its density becomes inappreciably small.

§ 329. Atmospheric air is 14.5 times as heavy as hydrogen. The pressure which the air exerts upon the surface of the earth at the sea-level is about, on an average, 15 lbs. to the square inch, and varies from about 14 lbs. to about 16 lbs. It supports a column of mercury 30 inches, or 760 millims. in height.

§ 330. the average composition of the atmosphere is as follows, by volume:

[blocks in formation]

The quantity of vapour of water in the air, however, is continually varying, and therefore influencing the whole amount of the other constituents, but without affecting their relative quantity. The proportion of N, O, and CO, varies very slightly in pure air. Collected at the tops of the loftiest mountains, during balloonascents, or at the sea-level, its composition is almost the same. There are generally in the air also traces of nitrous acid, ammonia, and marsh-gas. In towns, or crowded and ill ventilated rooms, the quantity of carbonic acid is greater.

§331. The composition of the atmosphere is maintained constant by the counteraction of numerous influences; these we must briefly examine. The flesh and tissues of animals are continually undergoing renovation; fresh matter is being carried to them through the arteries, and the waste, or no longer serviceable portion of them, is being carried away by the veins. It is carried to the lungs, and is there brought into contact with the inspired air, and becomes burnt or oxidized. Part of the products of this oxidation remain for a time in the blood and is got rid of by the kidneys; part is ejected at once in the breath. The waste compounds of carbon and hydrogen which are oxidized in the lungs to carbonic acid and water, escape as such in the expired breath. The well-known dampness of the breath is evidence of the water therein. If the breath be passed through lime-water, the lime is precipitated as carbonate of calcium, showing the presence of carbonic acid.

The air which is taken into the lungs on inspiration is found on expiration to be poorer in free oxygen, richer in carbonic acid and in watery vapour. Animal life is therefore a never-ending source of carbonic acid to the air. The immense quantities of wood, coal, gas, charcoal, coke, oil, fat, &c. burnt by man add a large amount of carbonic acid to the air. A very much larger quantity enters the air through the natural decay of wood, leaves, and other vegetable matter. Finally, a certain quantity is ejected from active volcanoes and mineral waters.

§ 332. This continual supply of carbonic acid is kept down by vegetation. Assisted by the light of the sun, the leaves of plants

have the power of reducing or deoxidizing carbonic acid, rejecting oxygen, and storing up the reduced carbon in combination with water, as wood, starch, gum, &c., all of which consist of carbon and hydrogen united with as much oxygen as would form water with the hydrogen.

CHO

It is true that at night, when the action of the sun has ceased, the leaves of plants give off carbonic acid, which is the product of the oxidation of the waste woody fibre and vegetable tissue. But the plant, as long as it is growing, effects on the whole a continual removal of carbonic acid from the air, and a storing up of the carbon in a less oxidized state. As far as its carbon is concerned, a plant may be said to breathe twice only in the twentyfour hours-one inspiration extending from the rising to the setting of the sun, during which the plant absorbs and digests the carbonic acid of the air, one expiration extending from the setting to the rising of the sun, during which the plant gives up a certain quantity of carbonic acid.

§ 333. It accordingly follows that, during the day, the expired breath of animals serves as food for plants, and the expired breath of plants contains additional oxygen fit for animals to respire, and that, during the night, plants and animals require the same constituents of the air, namely oxygen, and give rise to the same gas, carbonic acid. Hence the neighbourhood of plants is healthy for animals during the day, unhealthy during the night.

§ 334. Opinions are still divided as to whether plants may obtain their nitrogen directly from the free nitrogen of the air, whether they get it through the leaves from the small quantity of ammonia in the air, or whether it enters into the plant only through the roots, along with the metallic constituents required by the plant. The quantity of nitrogen in plants, however, is comparatively so small, that in any case the amount of nitrogen in the air cannot be appreciably affected by vegetable growth.

§ 335. The nitrous acid in the air, which may be detected in

the first shower following a summer thunderstorm, is supposed to be due to the direct union by the electric discharge of the free atmospheric nitrogen and oxygen. It is probably also formed by the oxidation of the minute germs of certain plants, which are known to exist in all air, and which doubtless contain nitrogen.

$336. Ammonia is probably mainly due to the putrefaction of nitrogenous animal and vegetable matter; or it may be formed simultaneously with nitrous acid-by the electric decomposition of water, and the union of both of its elements with nitrogen.

4H,0 + 4N = 2(NH ̧NO).

§ 337. The air of English towns usually contains traces of sulphurous and sulphuric acids. This is owing to the presence of sulphur in the coal used as fuel, its oxidation to sulphurous acid in the fire of the furnace, and the further gradual oxidation to sulphuric acid in the air.

$338. The analysis of atmospheric air is very easily performed. To determine the quantity of moisture in it, a known volume is made to bubble slowly through a weighed quantity of strong sulphuric acid. The sulphuric acid absorbs all the water, but does not affect the carbonic acid, the oxygen, or the nitrogen. If the air so dried be made to bubble through a known weight of a solution of hydrate of potassium, the carbonic acid is absorbed ; the oxygen and nitrogen pass through. If, finally, the mixed oxygen and nitrogen so deprived of water and carbonic acid be led over a weighed quantity of metallic copper heated to redness, the oxygen is absorbed, oxide of copper being formed. The increase in weight of the sulphuric acid gives the quantity of water; that of the hydrate of potassium gives the carbonic acid; that of the metallic copper gives the oxygen. The quantity of nitrogen is, of course, the difference between the weight of the quantity of air originally taken and the sum of the weights of the water, carbonic acid, and oxygen.

66

§ 339. The analysis of a small quantity of air is generally performed in what is called a eudiometer," which is a long glass tube closed at one end, and graduated or marked

in such a way that its contents from the closed end to any mark is accurately known. Two platinum wires are fused into the tube at the closed end so as nearly to touch one another in the inside of the tube (fig. 74). Such a tube is filled with

[blocks in formation]

mercury, and a little air is allowed to bubble up, and is measured by seeing how far it depresses the mercury. A small fragment of coke, A, which has been moistened with sulphuric acid and deprived of air in a barometric vacuum is then pushed up (by means of a wire fastened to it) into the gas, and after a time withdrawn. The decrease of volume thus effected gives the volume of the aqueous vapour. Secondly, a pellet of solid hydrate of potassium, B, is similarly introduced and withdrawn; the resulting contraction gives the volume of carbonic acid. Finally, a pellet of phosphorus, C, is introduced in a similar manner and allowed to remain for some hours. The diminution thereby caused is the volume of oxygen. The oxygen may be also determined by introducing a measured excess of hydrogen, and passing an electric spark through the mixed gases by means of the platinum wires; the diminution thus caused is equal to the volume of oxygen. Account must be taken in such analyses of the varying pressure and temperature at which the several measurements are made.

« PreviousContinue »