If, after the condition represented by the point C has been attained, the pressure is gradually decreased, the temperature meanwhile being maintained constant, the substance will expand along the isothermal CD, and the point D will be reached. As the volume occupied by the gas is further increased, the appearance of a well-defined meniscus will denote that vaporisation is occurring. The pressure meanwhile remains constant, the condition of the substance being successively represented by the points on the line DE. At E the substance is wholly converted into vapour.

A

Cagniard de la Tour's Experiment.-As early as 1822, Cagniard de la Tour performed an experiment on the conversion of a liquid into a gas, which we can now explain. Let us suppose that a volume, represented by the abscissa of the point F (Fig. 97), is occupied partly by a liquid and partly by its saturated vapour. If the temperature is raised, the volume remaining constant, the substance will pass through the conditions represented by the line FG. If the liquid originally occupied so small a fraction of the total volume, that its thermal expansion does not cause it to entirely fill the space before the critical temperature corresponding to curve 6 is attained, the substance will at that temperature pass from the liquid to the gaseous state. To realise these conditions experimentally, it is only necessary to fill a strong tube with liquid, boil about a third of this off, and then seal up. The tube can then be placed in a bath and heated.

B

FIG. 98.-Cagniard de la Tour's apparatus for determining the critical

tem

perature of a liquid. (P.)

The apparatus used by Cagniard de la Tour is represented in Fig. 98. This consisted of a long tube, one end of which, A, was filled with air to indicate the pressure, whilst the other end was bent round and fused on to a wide closed tube, B, containing the liquid and its saturated vapour. The air was separated from the liquid and its vapour by means of mercury, which filled the rest of the apparatus. When the whole was heated, the changes which took place were slightly more complicated than those previously explained by the aid of Fig. 97, since both the volume and

P

the pressure of the liquid and its saturated vapour were varied. However, it was observed that as the temperature was raised, the surface of the liquid gradually became flatter, indicating that the surface tension was diminishing. When a certain temperature was reached, the surface suddenly disappeared, so that the space above the mercury in B appeared to be filled with a homogeneous gas. This temperature was the critical temperature of the substance. The disappearance of the surface corresponded to the passage through G from the cross-hatched to the unshaded portion of Fig. 97.

EXPT. 53. To determine the critical temperature of sulphur-dioxide. -Take a piece of thick-walled thermometer tubing of about 2 mm. bore;

seal this at one end, and draw it out somewhat at a point A, Fig. 99, about 8 inches from the sealed end. Do not constrict the internal bore more than you can help in this process.

Take a piece of thin-walled glass tubing, and draw this out into a capillary tube, fine enough to pass down to the bottom of AB. Bend the capillary tube at right angles, and connect the wide tube in which it ends with the delivery tube of a bottle containing liquid sulphur-dioxide. Such bottles can be obtained from dealers in chemicals.

Take a wide test tube (or boiling tube), and having provided this with a cork bored to receive AB, Fig. 99, nearly fill it with a mixture of three parts (by weight) of broken ice to four parts of calcium chloride. Push AB (Fig. 99) through the hole provided in the cork, and arrange that the constricted part of the thermometer tube projects a little way above the latter. Place the capillary tube in position, and the arrangement will resemble that shown in section in Fig. 99.

On opening the valve of the bottle, liquid sulphur-dioxide will pass through the capillary tube and collect in the experimental tube. When this latter is nearly full close the valve and remove the capillary tube, and seal the experimental tube off at A by means of a small but very hot blowpipe flame. Some of the sulphur-dioxide will boil off during this process, but the tube should retain about two-thirds of its initial contents when the sealing has been effected.

Support the tube so that it is entirely immersed in a beaker about two-thirds full of glycerine, a thermometer reading to 200° C. being provided to indicate the temperature of the latter. Heat the glycerine by means of a Bunsen burner, and notice at what temperature the meniscus in the experimental tube disappears. Then allow the glycerine to cool, and note the temperature at which the meniscus once more becomes visible. The mean of these two temperatures may be taken as the critical temperature of sulphur-dioxide.

The pressure corresponding to the point G (Fig. 97) is termed the critical pressure of the substance.

Thus, it is impossible to liquefy a gas at a temperature higher than its critical temperature, and in order to liquefy it at that temperature a certain pressure, called the critical pressure, must be applied.

Pressure of Saturated Vapours.-It will be seen from Fig. 97, as well as from Andrews's curves, that when a vessel at a certain temperature is filled partly with liquid and partly with the vapour of that liquid, a certain definite pressure will be exerted by the vapour. This pressure corresponds to the ordinate of the horizontal straight line forming part of each isothermal for temperatures below the critical temperature. It is the greatest pressure which the vapour can exert at the given temperature, and is therefore termed the maximum vapour pressure (sometimes simply the vapour pressure) of the substance at the given temperature. The term "vapour tension" is also sometimes applied to the same value.

When the vapour pressure of a substance becomes equal to

the pressure of the atmosphere, bubbles of vapour are formed in the interior of the liquid, and ebullition occurs.

Liquefaction of Gases.-Such gases as ammonia, sulphurdioxide, &c., which should properly be classed with vapours, their critical temperatures being higher than the ordinary temperature of the atmosphere, may be liquefied either by merely subjecting them to high pressures, or by cooling them to low temperatures at atmospheric pressure. In other cases it is necessary not only to reduce the temperature below the critical value for the gas, but to apply a certain pressure. All known gases have now been liquefied, in most cases in large quantities. Some of the methods used, as far as these are related to the principles explained in this chapter, will be now described.

Faraday's Method. We may take the liquefaction of chlorine as typical of the methods employed by Faraday. The substances from which chlorine gas could be evolved were placed at one end of a strong bent glass tube, closed at both ends; the other end of this tube was immersed in a freezing mixture. The temperature of the freezing mixture being below the critical temperature of the gas, the pressure produced by the rapid evolution of the gas was sufficient to effect liquefaction.

This method was successfully employed by Faraday, in 1823, in the liquefaction of nitrous oxide, hydrochloric acid, cyanogen, chlorine, &c.

Liquefaction of Carbon-Dioxide. -In 1834 Thirlorier liquefied carbon-dioxide in the following manner. A strong copper cylinder, lined with lead, and strengthened with external iron bands, was filled to about a third of its height with bicarbonate of soda. Sulphuric acid was contained in an open tube placed in the cylinder (Fig. 100). The top being screwed on, the cylinder was inverted, when the acid became mixed with the bicarbonate of soda, producing a copious evolution of carbon-dioxide. The pressure produced is sufficient to liquefy the gas at ordinary temperatures. Referring to Andrews' curves, Fig. 96, it may be seen that a pressure of 50 atmospheres is sufficient for this purpose at a temperature of about 13° C.

The inside of the generating cylinder was then put in connection with the interior of another vessel, kept at a lower temperature. The carbondioxide distilled over into the latter, just as the water distils from one bulb to the other in Wollaston's cryophorus (p. 184).

When carbon dioxide is allowed to escape, under great pressure, through a narrow orifice into a metal vessel open to the atmosphere, the cold produced is sufficient to produce carbonic acid snow. This slowly

sublimes at a temperature of about 80° C. when exposed to the atmosphere. Mixed with ether, carbonic acid snow quickly evaporates, producing an exceedingly low temperature-about 77°C. This freezing mixture was used by Faraday in liquefying other gases.

Cailletet's Method (1877).-A cylinder A, Fig. 101, strong enough to withstand a pressure of 1,000 atmospheres, was provided with an air-tight piston joined to the end of a square-threaded screw B. An internal screw thread was cut in the hub of a large wheel, C, the rim of which was provided with spokes to facilitate turning. When the wheel was turned the piston was forced into the cylinder. The latter was filled with water, and the pressure obtained by forcing the piston inwards was transmitted by water, which filled flexible copper tubes of small bore, to the manometer M and the experimental tube T.