214 HISTOEY OF COLD AND THE ABSOLUTE ZERO. 



drawn between the use of the expressions zero of absokite tempera- 

 ture and the absohite zero. 



The w'hole question took an entirely new form when Lord Kelvin, 

 in 1848, after the mechanical equivalent of heat had been determined 

 by Joule, drew attention to the great principles underlying Carnot's 

 work on the Motive Power of Heat, and applied them to an absolute 

 method of temperature measurement which is completely independent 

 of the properties of any particular substance. The principle was that 

 for a difference of 1° on this scale between the temperatures of the 

 source and refrigerator, a perfect engine should give the same amount 

 of work in every part of the scale. Taking the same fixed points as 

 for the Centigrade scale, and making 100 of the new degrees cover 

 that range, it was found that the degrees not only within that range, 

 but as far beyond as experimental data supplied the means of com- 

 parison, differed by only minute quantities from those of Regnault's 

 air thermometer. The zero of the new scale had to be determined by 

 the consideration that when the refrigerator was at the zero of temper- 

 ature the perfect engine should give an amount of work equal to the 

 full mechanical equivalent of the heat taken up. This led to a zero of 

 273° below the temperature of freezing water, substantially the same 

 as that deduced from a stud}^ of the gaseous state. It was a great 

 advance to demonstrate by the application of the laws of thermody- 

 namics not only that the zero of temperature is a reality, but that it 

 must be located at 273° below the freezing point of water. As no one 

 has attempted to impugn the solid foundation of theory and experi- 

 ment on which Lord Kelvin based his thermodynamic scale, the exist- 

 ence of a definite zero of temperature must be acknowledged as a 

 fundamental scientific fact. 



LIQUEFACTION OF GASES AND CONTINUITY OF STATE. 



In these speculations, however, chemists were dealing theoretically 

 with temperatures to which the}^ could not make any but the most dis- 

 tant experimental approach. Cullen, the teacher of Black, had indeed 

 shown how to lower temperature by the evaporation of volatile bodies, 

 such as ether, by the aid of the air pump, and the later experiments of 

 Leslie and Wollaston extended the same principle. Davy and Faraday 

 made the most of the means at command in liquefying the more con- 

 densable gases, while at the same time Davy pointed out that the}^ in 

 turn might be utilized to procure greater cold l)v their rapid recon- 

 version into the aeriform state. Still the chemist was sorely hampered 

 by the want of some powerful and accessible agent for the production 

 of temperatures nuich lower than had ever been attained. That want 

 was supplied by Thilorier, wdio in 1835 produced li(|uid carbonic acid 

 in large quantities, and further made the fortunate discovery that the 

 li(|uid could be frozen into a snow ))v its own exaporation. Faraday 

 was prompt to take advantage of this new and potent agent. Under 



