530 HISTORY OF SCIENCE. 



The acceptance of the theory which regards heat as the motion of 

 the particles of bodies furnishes a remarkably direct and satisfactory 

 explanation of the facts of thermo-dynamics and of many physical pro- 

 perties of bodies, more especially of gases. The older conception of 

 a gas regarded it as constituted of material particles kept apart by 

 surrounding repulsive atmospheres of a highly elastic imponderable 

 fluid called caloric. The dynamical theory represents gases as formed 

 of particles moving freely in space according to the ordinary laws of 

 mechanics. The particles move in straight lines in all directions, with 

 doubtless different velocities, but their average velocity under denned 

 circumstances would also be definite. Such of the particles as en- 

 counter others will have the direction of the movement changed, but 

 the time of the moving forces of the whole will remain unchanged. 

 The same will be true of particles striking the wall of an enclosing 

 vessel. But the shocks of the impacts against the latter succeeding 

 each other with great rapidity and equally "in every direction, explain 

 the pressures exercised by gases. The increase of temperature when a 

 gas is made to occupy a smaller bulk when, for instance, being con- 

 tained in a cylinder, it is compressed by a piston is accounted for by 

 the communication of the movement of the piston to the particles. 

 The like, vice versa, is true in the case of a gas allowed to expand. 

 In fact, the dynamical theory explains with great readiness and clear- 

 ness not only Boyle's law (page 231), but the laws of the expansion by 

 heat, specific heats, diffusion, and other facts relating to gases. 



The laws according to which gases expand by heat have been the 

 subject of many elaborate experimental investigations. GAY-LUSSAC 

 (1778 1850), an eminent French chemist, summed up the results of 

 his experiments and those of others in the three following propositions, 

 which are often called Gay-Lussac's Laws : 



1. All gases expand equally by heat. 



2. 7 heir expansion is independent of the pressure. 



3. One volume of any gas taken at the freezing-point of water becomes 

 1*375 'volumes at the boiling-point. 



The subsequent experiments of M. REGNAULT (born 1810) showed 

 that these laws are true only approximately. All gases do not expand 

 alike, for the nearer to the degree of pressure and temperature at 

 which a gas becomes a liquid the experiment is made, the more ex- 

 pansible is the gas. Thus the expansion, instead of being '375 for 

 all gases, as stated by Gay-Lussac, was found by Regnault to be for 

 hydrogen '366, for air '367, for carbonic acid '371, and for sulphurous 

 acid gas '390. Faraday succeeded in liquefying by cold and pressure 

 many gases which had not previously been condensed, and the half- 

 dozen gases that had resisted all his efforts have quite recently been, 

 reduced to the liquid state by MM. Cailletet and Pictet. This result 

 is of importance, as it has completely proved the generality of the 

 recognized laws by actually removing the apparent exceptions. Some 



