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108. Physical properties of gases.—Gases, as we have already seen, are bodies whose molecules are in a constant state of repulsion, in virtue of which they possess the most perfect mobility, and are continually tending to occupy a greater space. This property of gases is known by the names expansibility, tension, or elastic force, from which they are often called elastic fluids.

The number of gases of which chemistry teaches us is very considerable ; but only four are elementary; these are oxygen, hydrogen, nitrogen, and chlorine. Some gases are coloured, but most of them are colourless. Some have a disagreeable odour, others are quite inodorous. Some are noxious, acting as poison to men and animals which breathe them: such are carbonic oxide, which is produced by the combustion of charcoal; sulphuretted hydrogen, which is given off from drains. Others are inoffensive, such as nitrogen and hydrogen ; yet an animal cannot live in them. They are not deleterious, in the sense of being poisonous ; but they do not support life. The only gas which has this property is oxygen; an animal deprived of this gas, even for a few seconds, soon dies.

Gases and liquids have several properties in common, and some in which they seem to differ are in reality only different degrees of the same property. Thus, in both, the particles are capable of moving ; in gases quite freely ; in liquids not quite freely, owing to a certain degree of viscosity. Both are compressible, though in very different degrees ; if a liquid and a gas both exist under a pressure of one atmosphere, and then the pressure be doubled, the water is compressed by about the 200000 part, while the gas is compressed by one-half. In density there is a great difference :

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water, which is the type of liquids, is about 800 times as heavy as air, the type of gaseous bodies, while under a pressure of one atmosphere. The property by which gases are distinguished from liquids is their tendency to indefinite expansion.

Matter assumes the solid, liquid, or gaseous form according to the relative strength of the cohesive and repulsive forces exerted between their particles. In liquids these forces balance: in gases repulsion preponderates.

By the aid of pressure and of very low temperatures, the force of cohesion may be so far increased in many gases that they are converted into liquids, and there is reason for believing that, with sufficient pressure and cold, they might all be liquefied. On the other hand, heat, which increases the force of repulsion, converts liquids, such as water, alcohol, and ether, into the aëriform state in which they obey all the laws of gases. This aëriform state of liquids is known by the name of vapour, while gases are bodies which, under ordinary temperature and pressure, remain in the aëriform state.

In describing the properties of gases we shall, for obvious reasons, have exclusive reference to atmospheric air as their type.

109. Atmospheric air.—Air is the gaseous fluid in which we live. It was regarded by the ancients as one of the four elements. Modern chemistry, however, has shown that it is a mixture of oxygen and nitrogen gases in the proportion of 20:8 volumes of the former to 79'2 volumes of the latter. By weight it consists of 23 parts of oxygen to 77 parts of nitrogen.

The oxygen feeds all the combustions which are produced round about us; and it also supports animal life. If it alone were present, or even if it were present in a larger proportion, the combustions would be too brisk, and life too active. The coal of our fireplaces would burn almost instantaneously, and even the grates in which it is contained would take fire. Life would be promptly consumed by so active a principle. The function of the nitrogen is to attenuate the too powerful effects of the oxygen.

Air is inodorous, transparent, and colourless, at any rate in small masses. In larger masses it is blue; thus arises the blue colour of the sky. Without air the celestial vault would appear black; it appears almost so when viewed from the tops of very high mountains, and from balloons ; for then the air above is very highly rarefied.

Air too, in virtue of its elasticity, is the medium for transmitting sounds; so that, if we were without it, the use of speech and of music would be lost.

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110. Expansibility of gases.—This property of gases, their tendency to assume continually a greater volume, is exhibited by means of the following experiment. A bladder closed by a stopcock, moistened so as to render it more flexible, and about half full of air is placed under the receiver of the air pump (fig. 87), and a vacuum is produced on which the bladder immediately distends. This arises from the fact that the molecules of air repel each other and press against the sides of the bladder. Under ordinary conditions this internal pressure is counterbalanced by the air in the receiver, which exerts an equal and contrary pressure. But when this pressure is removed by exhausting the receiver, the internal pressure becomes evident. When air is admitted into the receiver the bladder resumes its original form. The same effects would be produced what

Fig. 87. ever gases were contained in the bladder, thus showing that all are expansible.

111. Weight of gases. From their extreme fluidity and expansibility, gases seem to be uninfluenced by the force of gravity ; they nevertheless possess weight, like solids and liquids. To show this, a glass globe of 3 or 4 quarts capacity is taken (fig. 88), the neck of which is provided with a stopcock, which hermetically closes it, and by which it can be screwed to the plate of the air pump. The globe is then exhausted, and its weight determined by means of a delicate balance. Air is now allowed to enter, and the globe again weighed. The weight in the second case will be found to be greater than before, and if the capacity of the vessel is known, the increase will obviously be the weight of that volume of air.

By a modification of this method, and with the adoption of certain precautions, the weight of air

Fig. 88. and of other gases has been determined : 100 cubic inches of dry air under the ordinary atmospheric pressure of 30 in, and at the temperature of 16° C., weigh 31 grains; the same volume of carbonic acid gas under the same circumstances

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weighs 47.25 grains; 100 cubic inches of hydrogen, the lightest of all gases, weigh 2:14 grains; and 100 cubic inches of hydriodic acid gas weigh 146 grains.

The ratio of the density of air at C. and 30 inches pressure to that of water at o° C. is found to be o'001296. In other words, the latter is 771 times as heavy as the former.

112. The atmosphere. Experiments proving its weight.-The atmosphere is the name given to the layer of air which, like a light coating, surrounds our globe in every part. It partakes of the rotatory motion of the globe, and would remain fixed relatively to terrestrial objects, but for local circumstances, which produce winds, and are constantly disturbing its equilibrium.

The existence of this gaseous mass is proved by the winds, which incessantly blow on the surface of the earth; by the flight of birds, and the suspension of clouds.

Besides the oxygen and nitrogen of which the air is composed, the atmosphere also contains a quantity of aqueous vapour, which varies with the temperature, the season, the locality, and the direction of the winds. The mean amount of this in London is from 5 to 6 grains in a cubic foot of air.

It further contains from 3 to 6 parts in 10,000 of carbonic acid. This arises from the respiration of man and animals, from the decay of organic matter, and from the combustion of wood and coal. This latter cause of the production of carbonic acid increases every year. It has been calculated that in Europe alone about 104 milliards of cubic yards of carbonic acid are every year sent into the atmosphere from this source. This mass of gas is equal to what would be produced by 509 millions of individuals, each converting by the act of respiration 154 grains of carbon in their system into carbonic acid every hour.

Notwithstanding this enormous continual production of carbonic acid on the surface of the globe, the composition of the atmosphere does not vary ; for plants in the process of vegetation decompose the carbonic acid, assimilating the carbon, and restoring to the atmosphere the oxygen which is being continually consumed in the processes of respiration and combustion.

Thus, by a natural harmony, the atmosphere retains an almost uniform quantity of this gas, so that there is no fear of its accumulating to such an extent as to be injurious to the human species.

113. Atmospheric pressure.—Having seen that air has weight, it is easy to conceive that the great mass of air which constitutes

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the atmosphere must exert a great pressure on the surface of the earth, and on all bodies found there. This pressure is called the atmospheric pressure. It necessarily decreases as we ascend in the atmosphere ; for if we conceive the atmosphere resolved into horizontal layers superposed on each other, it is clear that the lower layers which support the weight of the whole atmosphere are the most compressed, and the most dense ; while the higher layers are less and less compressed and therefore less and less dense. This is expressed by saying that they are more rarefied or more rare. In saying that 100 cubic inches of air weighed 31 grains, it was understood that air at the sea level was referred to; at any greater height this volume would weigh less.

The pressure of the atmosphere may be demonstrated by a number of experiments, among which are the following:

114. Crushing force of the atmosphere.—On one end of a stout glass cylinder, about 5 inches high, and open at both ends, a piece of bladder is tied quite air-tight. The other end, the edge of which is ground and well greased, is pressed on the plate of the air-pump (fig. 89). The bladder is pressed downwards by the weight of the atmosphere, and is pressed upwards by the expansive force of the air in the cylinder. These two pressures at first counterbalance

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each other; the bladder is not pressed in either direction, but as soon as the internal air is removed from the vessel by working

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