expanded while doing work, i.e., behind a piston, a certain quantity of the heat it contained-the exact proportion depending upon the amount of work it is allowed to do - will disappear as work, and the temperature will fall. It is this property which makes the cold-air machine possible.

Free Expansion of Gases.-When, however, the pressure resisting the expansion of a gas is small, the gas is said to expand freely and does not perceptibly change its temperature. It has doubtless been noticed that when a refrigerating plant has been tested with air pressure, and the air is allowed to escape and expand from a pressure of, say, 300 lbs. per square inch to that of the atmosphere, there is no perceptible fall in its temperature; while if the same air was expanded while doing work in a cold-air machine or in a pneumatic drill or crane, its temperature would be many degrees below zero.

Mixtures of Gases.-Two or more gases, if brought together, will thoroughly mix if sufficient time be given. them. The rate of mixing depends upon the weight of the respective gases, and is inversely as the square root of their densities that is to say, a heavy gas like CO2 will take longer to mix with air than a lighter gas. This rule is equally true and holds good for a mixture of any number of gases.

Pressure of Gases.-The pressure exerted upon the interior walls of a vessel or cylinder containing a mixture of gases under pressure is equal to the sum of the pressures which would be exerted if each of the gases occupied the same vessel alone. This is known as Dalton's law.

Volume and Pressure of Gases.—If a gas is heated while its volume remains constant, its pressure will increase proportionally to the absolute temperature; while if it is

heated while its pressure remains constant, its volume increases according to the same law-that is, for a rise of temperature of 1° F. its volume increases by a constant fraction (1/492) of its volume at 32° F.

Unit of Pressure.-Pressure is measured in terms of the pressure of the atmosphere, which is taken as being equivalent to a pressure at sea-level of 147 lbs. per square inch. If this pressure of the atmosphere is removed by a vacuum pump and a vacuum formed, the vessel containing the vacuum is at once subjected to a pressure exerted upon it by the outside air in trying to flow in to destroy the vacuum. This property is made use of in suction. pumps; the air in the suction pipe being removed by the action of the pump plunger, the water is forced up the suction pipe by the pressure of the air outside, and, as a pressure of 14.7 lbs. on the square inch will support a column of water 32 feet high, the water, theoretically, will rise to this level in the suction pipe. In actual practice, however, a height or suction head of from 20 to 25 feet is the maximum obtainable, as, owing to the more or less imperfect valves of the water-pump, a better vacuum can seldom be had.

Liquefaction of Gases. All gases will become liquid, provided a sufficient pressure is applied and the temperature is sufficiently lowered. When gases or vapours are compressed, the energy or work of compression appears in the form of heat, and has to be removed before liquefaction will take place. In a compression refrigerating machine the heat removed by the condensing water is made up of the sum of the heat due to the energy of the steam-engine in compressing the gas and the heat that has been conveyed from the cold chamber or ice-tank by the refrigerating agent.

Critical Temperature and Pressure.

There is, however, a point called the critical temperature above which no amount of pressure will cause liquefaction. In the same way the pressure necessary to liquefy the gas at or near its critical temperature is known as the critical pressure. This latter term, however, is not so commonly heard as the former.

Isothermal and Adiabatic Expansion and Compression.When a gas is expanded or compressed while its temperature remains constant, heat being added or taken away, it is said to be expanded or compressed isothermally; but when, owing to no heat being taken away or added, the expansion or compression is accompanied by a change of temperature that is to say, the gas gets hotter or colder -the change is said to be adiabatic.

Absorption of Gases. All liquids, some more than others, will, if left to themselves, take a certain amount of gas into chemical combination with themselves. During this absorption heat is given off, and if the gas has afterwards to be separated from the liquid, an equivalent amount of heat to that given off has to be added from an external source. This is the principle of the ammonia-absorption machine, the ammonia and water being allowed to combine, and then the ammonia being driven off by the application of heat.

VAPOURS.

A vapour is merely a gas in a particular state, and is not a separate substance. When gases are lowered in temperature to their point of liquefaction, condensation begins to take place and they become charged with minute particles of liquid-in fact, become moist; in this state they are often called vapours. For instance, ordinary high and low pressure steam is a vapour, while superheated steam is a gas.

The term vapour is also applied to a gas below its critical point; thus, in this sense carbonic acid gas is a vapour at ordinary temperatures, while oxygen is not.

Saturated Vapours.-When the vapour is on the point of liquefying, it is said to be in a saturated state, and is then at its point of maximum density. Any vapour can, like steam, be superheated. It then becomes a gas, with

all the peculiarities of a gas. All liquids will, if exposed to the atmosphere, absorb heat from it and evaporate, part of the liquid becoming a vapour. In a closed vessel the evaporation will continue until the pressure in the vessel, due to the formation of the vapour, is equal to the maximum density of the vapour for the particular temperature at which evaporation is taking place. This evaporation only takes place from the surface of the liquid.

Boiling. When the temperature of a liquid becomes sufficiently high, vaporization will become rapid and take place from all points of the liquid, and what we know as boiling will occur. Bubbles of vapour will rise rapidly to the surface and there expand. The boiling-point of the various liquids varies, but boiling or ebullition will take place in any given liquid at a constant temperature for a constant pressure. If the vessel is a closed one, and the pressure is increased by the rapid formation of vapour, the temperature at which boiling will take place will rise, and as the pressure rises, more and more heat has to be supplied to the liquid before it will boil. If, on the other hand, the vessel is an open one, or so constructed that the vapour is drawn away or used as it is formed, as in a boiler supplying steam to a steam-engine, the temperature of the boilingpoint will remain constant until boiling stops. If the pressure is removed or decreased, the temperature at which the liquid will boil falls correspondingly, the lowest boiling

points being obtained under the influence of vacuum. When no special pressure is stated, it is always understood that the pressure of the atmosphere is meant. This question of the relation of pressure and temperature is most important; it is the key to the economical working of all refrigerating machinery, and as such should be thoroughly understood.

Latent Heat of Vaporization.—When heat is applied to a liquid in order to turn it into a vapour, part of the heat, as we have seen, remains latent, and is expended in overcoming the cohesion of the molecules of the liquid. This heat is known as the latent heat of vaporization, and varies considerably with the pressure at which vaporization takes place. As the boiling-point of different liquids varies, that of some being much higher than that of others, it stands. to reason that with a liquid having a boiling-point below the usual temperature of the atmosphere, the latent heat of vaporization, or the heat necessary to turn that liquid into a vapour, will be drawn from the objects surrounding it at the time of boiling. These objects, giving up their heat to the liquid, will fall in temperature and become cold or refrigerated, the amount of the refrigeration produced depending on the temperature at which the liquid is boiling. The vapour produced from the liquid contains not only the sensible heat, but also the latent heat of vaporization, and this heat has to be got rid of to some other object before the vapour can again become a liquid. If the boiling-point of the liquid is above the ordinary temperature of the atmosphere, this heat can be readily disposed of to the atmosphere or to water, and, as the boiling-point of liquid can be regulated at will within certain limits by the addition of pressure, all that is necessary, if the boiling-point is below the ordinary