THE 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[FOURTH SERIES.] 



AUGUST 1870. 



XI. Memoir on Internal Work in Gases. 

 By M. Achille Cazin*. 

 [With a Plate.] 

 Introduction* 

 "V\7'HEN the parts of a material system are connected by in- 

 . ▼ w ternal forces, any change in the arrangement of those 

 parts represents a certain sum of elementary mechanical opera- 

 tions produced in the interior of the system. 



The elementary work of a part is positive, if the displacement 

 of that part takes place in the direction of the internal force 

 which is applied to it, and negative in the inverse case. Hence 

 a positive internal work is the effect of internal forces acting as 

 motive forces ; and a negative internal work is the effect of these 

 forces acting as resistances. 



All known physical and chemical phenomena lead us to regard 

 a body as an assemblage of molecules ; and consequently we may 

 apply the preceding considerations to molecules connected by 

 internal forces. In solids and liquids the internal forces are not 

 simply attractive. A positive molecular work sometimes repre- 

 sents an increase of density, sometimes a diminution. When ice 

 melts, its density increases : now it is certain that here cohesion 

 diminishes, and that melting represents a negative internal work. 

 Hence the mutual action of two molecules of water is not a force 

 directed according to the line of their centres and depending 

 solely on their distance. The investigation of the sign of the 



* Translated from the Annates de Chimie et de Physique, January 1870, 

 having been communicated to the Academie des Sciences, March 9, 1868. 

 Phil. Mag. S. 4. Vol. 40. No. 265. Aug. 1870. G 



