PROGRESS IN PHYSICS. 319 



liquid and partly gas. He pointed out the fact that for the so-called 

 permanent g-a.se.s this critical temperature must be exceedingly low 

 and if such temperature could be reached liquefaction would follow. 



Subsequent progress in the liquefaction of gases came about by fol- 

 lowing this suggestion. Very low temperatures were produced by 

 subjecting the gas to great reduction in volume by pressure, remov- 

 ing the heat of compression by conduction and radiation, and then by 

 sudden expansion its temperature was greatly lowered. As early as 

 1877 two Frenchmen, Pictet and Cailletet, had succeeded in liquefy- 

 ing oxygen, hydrogen, nitrogen, and air. During the past twenty 

 years great improvements have been made in the methods of accom- 

 plishing these transformations, so that to-day it is eas}" to produce 

 considerable quantities of all of the principal gases in a liquid form, 

 and by carrying the reduction in temperature still further portions of 

 the liquid may be changed to the solid state. The most important 

 work along this line has been done by Wroblewski and Olszewski of 

 the University of Kracow, and Professor Dewar, of the Ro\'al Institu- 

 tion of London. Temperatures as low as about 250° C. below the 

 freezing point of water have been produced, the "absolute zero'' 

 being only 273° C below that point. These experiments promise to 

 throw much light on the nature of matter and are especiall}^ interest- 

 ing as revealing its extraordinary properties at extremely low tem- 

 peratures. Among the most curious and suggestive is the fact that 

 the electrical resistance of pure metals diminishes at a rate which indi- 

 cates that at the absolute zero it would vanish and these metals would 

 become perfect conductors of electricity. 



The dynamics of heat, or •' thermodynamics,"' was an important field 

 of research in the early part of the century, on account of its practical 

 application to the improvement of the steam engine. The science was 

 created by Carnot, who, in spite of the fact that his views regarding 

 the nature of heat were erroneous, discovered some of the most inter- 

 esting relations among the quantities involved and discussed their appli- 

 cations to the heat engines with great skill. Subsequent contributors 

 to the theory and practice of thennodynamics were Clausius, Kankine, 

 Lord Kelvin, and Professor Tait. 



The mechanical theory of heat naturally led up to what has already 

 been referred to as the most important generalization in physical sci- 

 ence since the time of Newton — the doctrine of 



THE CONSERVATION OF ENERGY. 



This principle puts physics in its relation to energy where chemistry 

 has long been in its relation to matter. If matter were not conserva- 

 tive, if it could be created or destroyed at will, chemistry would be an 

 impossible science. Physics is put upon a solid foundation by the 

 assumption of a like consei-vatism in energy; it can neither be created 



