678 HEAT the temperature it had before compression ; but if, while under compression, it is allowed to cool to its previous temperature, and the pressure is then removed, it will fall through as many degrees as it had been raised by compression. Upon the principle here in- volved, gases which were formerly considered permanent have been reduced to a liquid and to a solid condition. Faraday employed the following method: Introducing materials for producing a gas in one end of a bent tube, fig. 14, and heating it previous to their com- bination, and then applying a gentle heat, a vast pressure was produced by the gener- ated gases, and then by placing the other end of the tube in a freezing mixture, condensation was effected. Thilorier in 1834 constructed on this principle an apparatus which was ca- pable of liquefying large quantities of carbonic acid gas. The operation requires a pressure of about 50 atmospheres, or about 700 Ibs. to the square inch. The vessels were formerly made of cast iron, strengthened with wrought- FIG. 15. Thilorier's Apparatus for Liquefying Carbonic Acid. iron hoops; but explosions occurring, attended with loss of life, the construction was modified by using leaden vessels surrounded with copper ones, bound with strong iron hoops. The ap- paratus is represented in fig. 15, and consists of two vessels, one a condenser and the other a generator, the latter being -represented in section. Bicarbonate of soda is placed in the generator, and also a cylindrical vessel contain- ing sulphuric acid. The generator being sup- ported by pivots, it can be turned to spill the acid. The resulting gas, evolved in large quan- tities, is forced through the connecting tube into the condenser, which is surrounded by a freezing mixture, and is there condensed into a liquid. When some of the liquefied gas is allowed to escape into the air, a portion ex- pands into gas, which so chills the remainder that it solidifies and forms white flakes, like snow, its temperature being about 129 F. ; and if this is mixed with ether, the cold which is produced is so intense as to have an effect upon the skin like that of burning with hot iron. By placing this mixture in the exhausted receiver of an air pump, Fara- day caused the temperature to fall to 166 below zero ; and M. Natterer by the use of a bath of nitrous oxide and bisulphide of carbon, previously liquefied by cold and pressure, low- ered the temperature to 220 below zero ; and Despretz succeeded in reducing alcohol to a viscous state. Liquid carbonic acid contained in a tube and placed in this mixture instant- ly becomes solid, assuming the appearance of transparent ice. By the use of this mixture and very high pressure, Andrews reduced air to -5^-5- of its original volume, oxygen to ^ f1 hydrogen to yi^, carbonic oxide to 7 fg-, and nitric acid to ^^, but without producing lique- faction. There was some departure from Ma- riotte's and Boyle's law (see ATMOSPHEBE), but it was less in hydrogen and carbonic oxide than in the other gases. Freezing on a large scale by Carre's apparatus, described in the article FKEEZINO, is effected on the principle of absorption of heat by evaporation and ex- pansion. The absorption of heat by lique- faction has a familiar example in the ordinary freezing mixture of snow or pounded ice arid common salt, by which the zero tempera- ture of Fahrenheit's thermometer was ob- tained. An interesting experiment in the ab- sprption of heat by liquefaction and its reap- pearance on solidification is made by dissolving sulphate of soda in water. The two. being mingled at the same temperature, the thermom- eter will indicate a fall. If the solution is warmed and saturated, and then allowed to cool while perfectly at rest, a point will be reached at which more of the soda will re- main in solution than could have been dis- solved at the same temperature. The polar relations of the molecules of the salt by which solution is maintained require, in order that solidification may take place, to be dis- turbed by a further reduction of temperature or by a mechanical impulse. The condition of solution is maintained by an expenditure of energy which when solidification or crystalli- zation takes place resumes the condition or motion of heat. Agitation of the vessel, or of its contents by dropping among them a crystal of the salt, will cause crystallization to com- mence ; and the bulb of a thermometer plunged into the mass will show a rise of temperature. Prof. James Thomson, in a paper published in the " Transactions of the Eoyal Society of Edinburgh" in 1849, expressed his opinion, de- duced from the mechanical theory of heat, that a liquid which expands in solidifying, like water, must have its melting point lowered by increase of pressure. Sir William Thomson soon after tested the question by experiment, and proved the correctness of the deduction. When a mixture of ice and water was subject- ed to pressure, the temperature fell, returning again to 32 when the pressure was removed. The addition of pressures of 8'1 and 16*8 at- mospheres lowered the freezing point 0*106 and 0-232 F. respectively ; results which very
