266 RUDOLF JULIUS EMANUEL CLAUSIUS. 



some of the most distinguished names among his contemporaries. 

 Somewhat later (1870) Clausius, whose attention had not been called 

 to Boltzmann's work, wrote his paper, " Ueber die Zuriickf tihrung des 

 zweiten Hauptsatzes der mechanischen Warmetheorie auf allgemeine 

 mechanische Principien."* 



The point of departure of these investigations, and others to which 

 they gave rise, is the consideration of the mean values of the force- 

 function and of the vis viva of a system in which the motions are 

 periodic, and of the variations of these mean values when the external 

 influences are changed. The theorems developed belong to the same 

 general category as the principle of least action, and the principle 

 or principles known as Hamilton's, which have to do, explicitly or 

 implicitly, with the variations of these mean values. 



Among other papers of Clausius on this subject, we may mention 

 the two following: "Ueber einen neuen mechanischen Satz in 

 Bezug auf stationare Bewegung"t (1873), and "Ueber den Satz vom 

 mittleren Ergal und seine Anwendung auf die Molecularbewegungen 

 derGase": (1874). 



The first problem of molecular science is to derive from the observed 

 properties of bodies as accurate a notion as possible of their molecular 

 constitution. The knowledge we may gain of their molecular con- 

 stitution may then be utilized in the search for formulas to represent 

 their observable properties. A most notable achievement in this 

 direction is that of van der Waals, in his celebrated memoir, " On the 

 Continuity of the Gaseous and Liquid States." To this part of the 

 subject belong the following papers of Clausius: "Ueber das Verhalten 

 der Kohlensaure in Bezug auf Druck, Volumen und Temperatur," 

 and " Ueber die theoretische Bestimmung des Dampfdruckes und der 

 Volumina des Dampfes und der Fliissigkeit" (two papers). || 



Another matter in which Clausius showed his originality and 

 power was the vexed subject of electrodynamics, as treated in his 

 memoir, " Ueber die Ableitung eines neuen electrodynamischen 

 Grundgesetzes." IF Various points in the theory of electricity in 

 which the principles of thermodynamics or of molecular science 

 were involved, had previously been treated in different papers, of 

 which the earliest appeared in 1852,** while the doctrine of the 



* Pofjg. Ann., vol. cxlii, p. 433. 



Mbid., vol. cl, p. 106. 



%Ibid., Ergdnzungsband vii, p. 215. 



Wied. Ann., vol. ix, p. 337 (1880). 



|| Ibid., voL xiv, p. 279 and p. 692 (1881). 



ICrdle's Journal, vol. Ixxxii, p. 85 (1877). 



** " Ueber das mechanische Aequivalent einer electrischen Entladung und die dal 

 stattfindende Erwarmung des Leitungsdrahtes. " Pogg. Ann., vol. Ixxxvi, p. 337. 

 " Ueber die bei einem stationaren electrischen Strome in dem Leiter gethane Arbeit 



