477. The passage from the liquid to the gaseous state is usually accompanied by a large increase of volume. Thus a cubic inch of water is converted by boiling into about 1700 cubic inches of steam; so that the cubic inch of water becomes nearly a cubic foot of steam. If we suppose that the substance consists of a large number of particles, placed at nearly equal distances, then we may imagine that in passing from the state of water to that of steam the average distance between two adjacent particles becomes about twelve times as great as at first.

478. The changes of state take place at different temperatures for different bodies. Thus to cause water to take the solid state of ice the temperature must be reduced to 32 degrees of Fahrenheit's thermometer; while if the temperature is raised to 212 degrees the water becomes steam. Mercury freezes at about 40 degrees below the zero of Fahrenheit's thermometer, and boils at about 650 degrees. Thus we see that one substance, as mercury, may remain in the liquid state at a temperature so low that another substance, as water, becomes solid; and at a temperature so high that another becomes gaseous.

479. There is a curious fact connected with the passage of bodies from the solid to the liquid state, and from the liquid to the gaseous; it is expressed by the statement that in these changes heat becomes latent. Suppose that a pound of water at 32 degrees of heat as measured by Fahrenheit's thermometer is mixed with a pound of water at 174 degrees; it is found that the temperature of the mixture is 103 degrees which is half the sum of 32 and 174 degrees: the hotter water has lost, and the colder has gained 71 degrees of temperature. But now suppose that a pound of ice at 32 degrees is put with a pound of water at 174 degrees; after a time the ice will all be melted, and the temperature of the mixture will be only 32 degrees. The water has lost 142 degrees of temperature, and the ice has been melted without any apparent increase of temperature: the heat thus lost by the water is said to be latent in the melted ice. Thus we see that in the process of converting a solid into a liquid a large quantity of heat is required which is in some manner absorbed by the liquid and does not become apparent by a rise of temperature.

In like manner when water is converted into steam a large quantity of heat becomes latent. The amount is greater than in the case of liquefied ice, being now about 900 degrees instead of 142. These numerical values have been differently assigned by various experiments; but extreme accuracy is unnecessary for our purpose.

480. The term latent heat has been long in use and perhaps does not often lead to any confusion or error; but there is always a danger that such descriptive terms should be made to suggest more than they are actually intended to convey. It might be objected in the present case that heat cannot be properly said to be hidden because its influence is manifested in the remarkable change of state, namely, from the solid to the liquid state, or from the liquid to the gaseous.

481. Common air is the most obvious and the most important of the gaseous bodies, and we shall in the main confine ourselves to the properties of air, though the mechanical results obtained are applicable in general to gases and vapours. The science which relates to the mechanical properties of the air is called Pneumatics. It belongs to Chemistry to treat of the special properties of each gas.

XLII. AIR A SUBSTANCE.

482. The atmosphere is a thin fluid which surrounds the globe, and is necessary for the support both of animal and vegetable life. Although before attention has been drawn to its properties it might be imagined that air is scarcely a form of matter, yet on due consideration it will be found to be such, though in a very rarefied condition.

483. The air is generally supposed to be transparent, but when we look at a cloudless sky we recognise a blue colour which may be attributed to the air. The fact that this colour is not visible when we inspect a small quantity of air by itself is consistent with other facts of a similar kind. Thus sea-water in a large mass presents a greenish tint, but a small quantity of it seems without colour. So also wine in a very slender glass appears much paler in tint than in a wider glass.

484. One of the most obvious properties of matter is weight, and air may be shewn to possess this. It would seem a natural process to test this by first weighing an empty bladder, and then weighing this bladder full of air; Aristotle is said to have done so, and, finding the same result in the two cases, to have inferred that air has no weight. But here we have the operation of a cause of error to which we drew attention in Art. 444; the additional weight of air in the bladder is counterbalanced by the buoyancy of the atmosphere exerted on the inflated bladder. The experiment must then be made in a manner which avoids this cause of error. Take a flask of glass or metal, and exhaust it of air by the aid of a machine to be described hereafter, called the air-pump; then weigh the exhausted flask. Admit the air to the flask and weigh it again. Then the difference between the two results gives us the weight of the air which the flask will hold. As we have said in Art. 462 the weight of a cubic foot of air under ordinary circumstances is about 1 ounces. We spoke of exhausting the flask of air; but in practice we cannot draw out all the air, though we may contrive to leave only a quantity which is quite inappreciable. Again, the experiment may be carried a step further. For not only can we draw air out of a vessel, but we can force into it any quantity of air we please. Thus we can increase the amount of air in the vessel, and we shall find that as we do so we increase the weight of the air in the same proportion.

485. Again, the resistance which air opposes to motions through it is an evidence that it has the properties of matter; we are very sensible of this resistance when we run. The reaction of the air when they strike it with their wings enables birds to fly; in a space void of air they could not fly. Wind is air in motion, and the powerful effects of high winds are merely the consequences of matter in violent motion.

486. It is usual to remark that air possesses the property of matter which we call impenetrability. Invert a tumbler and press it below the surface of water; then it is easy to see that the water does not get to the highest part

of the tumbler. If a small cork be floating on that part of the water over which the tumbler was placed, the cork will not reach the highest part of the tumbler. The air in

the tumbler is indeed compressed into less space than it originally occupied, and so the water occupies part of the. tumbler; but the air remains in the upper part of the tumbler and excludes the water from it.

XLIII. PRESSURE OF THE ATMOSPHERE.

487. If we put air in a vessel furnished with a moveable piston we find that we can push in the piston and compress the air to any extent we please. If we wish to keep the air in this compressed state we must retain the piston in its place by a suitable force; if we diminish that force the air pushes the piston back through some space, and if we remove all the force the air resumes its original dimensions. There must be some relation then between the force which we apply to the piston, and the volume occupied by the compressed air; this relation we shall consider in the next Chapter after some necessary preliminaries in the present.

488. We know that air requires the exercise of some constraint to confine it within the space it occupies, and so we naturally suppose that there must be some pressure acting on the apparently unconstrained air around us, and we soon find that this pressure must be supplied by the atmosphere itself; any stratum of air has to support the pressure produced by the weight of all the strata above it. A very important experiment serves to demonstrate the existence of the pressure of the atmosphere, and to measure its amount.

489. To measure the pressure of the atmosphere.

Take a glass tube a yard long, open at one end and closed at the other; fill it with mercury and place a finger over the open end to prevent the escape of the mercury. Invert the tube, put the end closed by the finger below the surface of a vessel containing mercury, and withdraw the finger. Some of the mercury will fall out of the tube,

A

leaving a vacuum, that is an empty space, at the top of the tube. In the diagram let AB denote the tube, EF the surface of the mercury in the vessel, and G the surface of the mercury in the tube. It is found. that the height of G above the level of EF is about 30 inches, so long as the place at which the experiment is made is not much above the level of the sea; but even at the same place the height is always fluctuating slightly according to the state of the temperature and the weather. The column of mercury above the level of EF is supported by the pressure of the atmosphere on the surface of the mercury in the vessel; this pressure is transmitted through the mercury in the vessel, and into the tube by means of the end B. The principle is the same as in Art. 420; we may imagine two tubes, one containing mercury of about 30 inches high, and the other extending upwards as far as the atmosphere extends, and the columns of mercury and of air would produce the same pressure at their lowest points: the column of mercury must be supposed to be in a tube closed at the top so as to relieve it from the pressure of the atmosphere above it.

E

B

D

490. As we ascend to a height above the level of the sea the pressure of the atmosphere diminishes, and so the height of the column of mercury diminishes. If the atmosphere were throughout of the same density there would be a diminution of about one inch in the mercury for every 900 feet of ascent; but the fact is that the higher we ascend the less is the density of the atmosphere, and so the diminution of the column of mercury is not in exact proportion to the ascent.

491. We see then that the pressure of the atmosphere under ordinary circumstances on a square inch of surface is equal to the pressure of a column of mercury of the height 30 inches standing on one square inch as base: thus