Subsequent progress in the liquefaction of gases came about by following this suggestion. Very low temperatures were produced by subjecting the gas to great reduction in volume by pressure, removing 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 liquefying oxygen, hydrogen, nitrogen, and air. During the past twenty years great improvements have been made in the methods of accomplishing these transformations, so that to-day it is easy 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 Cracow, and Professor Dewar, of the Royal Institution in London. Temperatures as low as about two hundred and fifty degrees C. below the freezing-point of water have been produced, the “absolute zero” being only two hundred and seventy-three degrees C. below that point. These experiments promise to throw much light on the nature of matter, and they are especially interesting as revealing its extraordinary properties at extremely low temperatures. Among the most curious and suggestive is the fact that the electrical resistance of pure metals diminishes at a rate which indicates that at the absolute zero it would vanish, and these metals would become perfect conductors of electricity. The dynamics of heat, or “thermo-dynamics,” 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 interesting relations among the quantities involved, and discussed their applications to the heat engines with great skill. Subsequent contributors to the theory and practice of thermo-dynamics were Clausius, Rankine, 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 science 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 conservative, 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 conservatism in energy; it can neither be created nor destroyed, although it may appear in many different forms which are, in general, mutually interconvertible.