318 PROGRESS IN PHYSICS. 



and Carnot. Fourier wrote especially of the movement of heat in 

 solids, and as his thesis depended in no wa}" on the nature of heat it 

 will alwaj^s be regarded as a classic. His assumption that conduc- 

 tivity was independent of temperature was shortly proved to be 

 erroneous, but his general argument and conclusions were not greatly 

 ati'ected bv this discovery. His work is one of the most beautiful 

 examples vet produced of the application of mathematics to physical 

 research, and mathematical and physical sciences were equally enriched 

 b}" it. In its broader aspects his law of conduction includes the trans- 

 fer of electricity in good conductors and is the real basis of Ohm's law. 



One of the most skillful and successful experimenters in heat was 

 also a Frenchman, Henri Victor Kegnault (1810-1878). He greatly 

 improved the construction and use of the thermometer, and was the 

 tirst to fliscover that the indications of an air thermometer and one of 

 mercury did not exactly agree, because thej^ did not expand in the same 

 degree for equal increases of temperature. His most important work 

 was on the expansion of gases, vapor pressure, specific heat of water, 

 etc., and for careful, patient measuring he had a positive genius. 

 Until he proved the contrary it had been assumed that all gases had the 

 same coefficient of expansion, and Boyle's law that the volume of a gas 

 was inversel}' proportional to its pressure had not been questioned. 

 His tables of the elastic force of steam have been of immense practical 

 value, but his studies of the expansion of gases are of greater interest, 

 because they have pointed the way to one of the most important 

 accomplishments of the century, the liquefaction of all known gases. 



During the earlier years of this century it was the custom to con- 

 sider vapors and gases as quite distinct forms of matter. Vapors 

 always came, b}^ evaporation, from liquids and could alwaj^s be "con- 

 densed" or reduced to the liquid form without difficulty, but it was 

 not thought possible to liquefy the so-called "permanent" gases. The 

 first man to attack the problem systematically was Michael Faraday, 

 who before the end of the tirst third of the century had liquefied sev- 

 eral gases, mostly by reducing them by chemical reactions under pres- 

 sure. Several of the more easily reducible gases or vapors, such as 

 ammonia, sulphurous acid, and probably chlorine, had been previously 

 liquefied by cold; but a quarter of a century elapsed after Faraday's 

 researches before the true relation of the liquid and gaseous states of 

 matter was understood, and it was found that both increase of pres- 

 sure and lowering of temperature were, in general, essential to the 

 liquefaction of a gas. It was Thomas Andrews, of Belfast, who first 

 showed in a paper published in 1863 that there was a continuity in 

 the liquid and gaseous states of matter; that for each substance there 

 was a critical temperature at which it became a homogeneous fluid, 

 neither a liquid nor a gas; that above this temperature great pressure 

 would not liquefy, while below it the substance might exist as partly 



