451. Gas and Vapor.-It thus appears that there is no sharp distinction between gases and vapors. What are ordinarily known as gases are substances whose critical temperatures are so low or critical pressures so great that under ordinary conditions they cannot exist as liquids with a free surface, while vapors arise from substances whose critical temperatures are so high that they ordinarily exist even at atmospheric pressure in the liquid state.

Critical Temperatures, Pressures, and Volumes.

452. Condensation of Gases.-Faraday, about 1823, began a series of experiments in which he liquefied nearly all the known gases except oxygen, nitrogen, hydrogen, and carbon monoxide. The form of apparatus used by him for chlorine and other gases is shown in figure 242. It consists

FIG. 242.

Ice

of a strong bent glass tube hermetically sealed, in one end of which is placed the substance or mixture from which the gas is to be evolved, while the other end is placed in a freezing mixture to induce condensation. When heat is applied the gas is given off on one side and the pressure due to its own evolution causes it to condense on the other. In cases where heat was not required the two ingredients were placed separately in the two branches of the tube which was then sealed. The tube was then tipped up so that the substances were mixed in one branch of the tube while the other was introduced into the freezing mixture as before.

Solid Carbon Dioxide.-When carbon dioxide, after being liquefied by pressure, is cooled, and then allowed to escape from a small opening, the evaporation and expansion produce such a degree of cold that a considerable part of the escaping substance is frozen into snow. If the jet is enclosed in a woolen bag this snow may be collected. It slowly sublimes, passing directly from the condition of solid to vapor only so fast as the necessary heat of vaporization is obtained from surrounding bodies.

The temperature of solid CO, at atmospheric pressure is -78° C.; if a little is placed on the hand it is kept from close contact at first by the gas given off due to the heat of the hand. If pressed into contact it burns like a hot iron. If mixed with ether a freezing mixture is obtained giving a temperature of -78° C. at atmospheric pressure, and if the pressure is reduced by an air pump - 116° C. may be reached. The mixture even at

atmospheric pressure readily freezes mercury.

453. Liquefaction of Air.-Many attempts were made to liquefy the permanent gases, as they were called, by Faraday, Natterer, and others, but without success until, in 1878, Cailletet and Pictet, working independently, one at Paris and the other at Geneva, almost simultaneously achieved the desired result. Both subjected the gases to great pressure and then cooled them to the lowest point attainable by evaporating liquid sulphur dioxide or carbon dioxide under diminished pressure. But even at the low temperatures thus secured no condensation was observed until a stopcock was opened and the compressed gas suddenly permitted to expand. The cooling due to this sudden expansion caused a cloud of particles of condensed gas to appear. In this way oxygen, nitrogen, and carbon monoxide were shown to be liquefied.

WROBLEWSKI and OLSZEWSKI obtained a still lower temperature by cooling liquid ethylene first with ice and salt, then with carbon-dioxide snow mixed with ether, and finally the cooled ethylene contained in a triple-walled glass vessel was made to boil at diminished pressure, the gas as it evaporated being pumped out by an exhaust pump. (Fig. 243.) In this way a temperature of 136° C. was reached, and when oxygen was compressed into a tube dipping below the surface of the boiling ethylene it was condensed at a pressure of about 20 atmospheres.

In this manner considerable quantities of liquid oxygen and nitrogen were first obtained.

LINDE'S APPARATUS. The present methods of obtaining liquid air on a large scale are based on the progressive cooling of a stream of escaping gas by its own expansion, and the first apparatus of this kind was devised by Dr. Linde in 1895. Compressed air at a pressure of about 200 atmospheres, and dried and purified from carbon dioxide, passes into the inner tube of the interchanger which contains long coils of tubing one within the

other and packed in felt to prevent the inflow of heat from outside. At the lower end of the interchanger is a needle valve through which the compressed gas is allowed to escape in a steady stream. The escaping gas cooled by expansion passes out through the outer tube of the interchanger, thus cooling the inflowing stream of compressed gas in the inner tube. But this on expansion is still further cooled and so the cooling goes on progressively until the temperature becomes so low that a part of the air is liquefied as it escapes at the valve and falls into the receiver below, where it is collected. (See note $455.)

This receiver is a double-walled glass vessel (such as are used in thermos bottles) known as a Dewar flask. The space between the walls of the flask is thoroughly exhausted of air to prevent

conduction of heat to the inner vessel. If a drop of mercury is contained in the vacuum space it will condense over the surface of the inner vessel forming a bright metallic mirror that reflects radiation and thus still further aids in preventing the liquid air from receiving heat from outside.

Liquid air when first produced contains both oxygen and nitrogen, but as the boiling point of nitrogen (-195.5° C.) is lower than that of oxygen (-183° C.) the former soon boils off leaving nearly pure oxygen.

454. Liquefaction of Hydrogen and Helium.-The liquefaction of hydrogen has been accomplished by Dewar using an improved form of Linde's apparatus in which two separate interchangers were used, one entirely surrounded by the other. The outer one was first used for the production of liquid air and in this way the whole apparatus was cooled to 180° C. Hydrogen was then passed through the inner apparatus and still further cooled by its own expansion until it finally collected as a clear liquid boiling under atmospheric pressure at -252° C. or 21 degrees above the absolute zero.

On reducing the pressure the temperature of the boiling hydrogen was lowered until it froze into a solid at -258° C.

A cubic centimeter of liquid hydrogen weighs .086 gram; it is therefore the lightest liquid known.

If a bulb containing air has a long neck which is sealed up and surrounded by liquid hydrogen the air will condense and freeze in the neck leaving the bulb highly exhausted.

If fragments of box charcoal are contained in the cooled neck their absorption is so powerful that the bulb becomes almost a perfect vacuum.

Helium, the last gas to yield to condensation, was finally liquefied in 1908 by the Dutch physicist Onnes. Liquid helium, according to Onnes, boils at -268.5° C., and has a density 0.15.

455. Note on Cooling by Expansion in Linde's Apparatus.The cooling of a steady stream of gas escaping under pressure through a small opening, as in Linde's apparatus for the liquefaction of air, is by no means as great as when a mass of gas is expanded in a nonconducting cylinder as explained in §413. For while the expansion of the gas tends to cool it, the kinetic energy of the gas rushing out of the opening tends to heat the expanded gas and one effect nearly balances

the other so that the cooling is but slight in case of air at room temperature. As air is cooled, however, the effect increases and when a sufficiently low temperature is reached to liquefy the air, the latent heat of vaporization is involved just as in an ammonia refrigerating machine. When a stream of hydrogen gas at room temperature is forced in this way through a small opening the gas is slightly heated instead of being cooled and it is only after being cooled below -80° C. to begin with, that the escaping jet is cooled at all by expansion.

456. Heat Engines.-The conversion of heat into mechanical energy is of the greatest importance to man, since vast stores of fuel existing in the earth as coal, petroleum and gas are thus made available for useful work.

It is interesting to consider that these deposits are really storehouses of the energy of sunlight which fell on the earth in ages long gone by and effected the separation of carbon from oxygen in plants, thus storing up potential energy which is ready to be given back to us as energy of heat under the magic touch of flame.

The principal kinds of heat engines are the steam engine, the hot-air engine, and engines that burn gas or vapors explosively.

457. The Steam Engine. A simple double-acting steam engine is shown in the diagram (Fig. 245). Steam from the boiler is admitted to the steam chest S, passes through one steam port into the cylinder A and forces the piston toward the left. Whatever steam or air is in the other end of the cylinder B escapes through the other port, passes under the cup-shaped slide valve and out at the exhaust E. But the slide valve is so con