ADDRESS. 25 



as experimental data supplied the means of comparison, 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 temperature 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 degrees below the temperature 

 of freezing water, substantially the same as that deduced from a study 

 of the gaseous state. It was a great advance to demonstrate by 

 the application of the laws of thermodynamics not only that the zero of 

 temperature is a reality, but that it must be located at 273 degrees below 

 the freezing-point of water. As no one has attempted to impugn the 

 solid foundation of theory and experiment on which Lord Kelvin based 

 his thermodynamic scale, the existence 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 they 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 WoUaston 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 they in turn 

 might be utilised to pi'ocure greater cold by their rapid reconversion into 

 the aeriform state. Still the chemist was sorely hampered by the vf&ni 

 of some powerful and accessible agent for the production of temperatures 

 much lower than had ever been attained. That want was supplied by 

 Thilorier, who in 1835 produced liquid carbonic acid in large quantities, 

 and further made the fortunate discovery that the liquid could be frozen 

 into a snow by its own evaporation. Faraday was prompt to take ad- 

 vantage of this new and potent agent. Under exhaustion he lowered its- 

 boiling-point from minus 78° C. to mimis 110° C, and by combining this 

 low temperature with pressure all the gases were liquefied by the year 

 1844, with the exception of the three elementary gases — hydrogen, nitro- 

 gen, and oxygen, and three compound gases — carbonic oxide, marsh gas, 

 and nitric oxide ; Andrews some twenty-five years after the work of 

 Faraday attempted to induce change of state in the uncondensed gases, 

 by using much higher pressures than Faraday employed. Combining the 

 temperature of a solid carbonic acid bath with pressures of 300 atmospheres, 

 Andrews found that none of these gases exhibited any appearance of 

 liquefaction in such high states of condensation ; but so far as change of 

 volume by high compression went, Andrews confirmed the earlier work 

 of Natterer by showing that the gases become proportionately less 

 compressible with growing pressure. While such investigations were 



