Page:Scientific Papers of Josiah Willard Gibbs - Volume 2.djvu/282

266 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 Zurückführung des zweiten Hauptsatzes der mechanischen Wärmetheorie 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: "eber einen neuen mechanischen Satz in Bezug auf stationäre Bewegung" (1873), and "Ueber den Satz vom mittleren Ergal und seine Anwendimg auf die Molecularbewegungen der Gase" (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 constitution 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 Kohlensäure in Bezug auf Druck, Volumen und Temperatur," and "Ueber die theoretische Bestimmung des Dampfdruckes und der Volumina des Dampfes und der Flüssigkeit" (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." 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