Page:Scientific Papers of Josiah Willard Gibbs - Volume 2.djvu/281

Rh development naturally commenced with the simplest case involving the characteristic problems of the subject,—the case, namely, of gases. The origin of the kinetic theory of gases is lost in remote antiquity, and its completion the most sanguine cannot hope to see. But a single generation has seen it advance from the stage of vague surmises to an extensive and well established body of doctrine. This is mainly the work of three men, Clausius, Maxwell, and Boltzmann, of whom Clausius was the earliest in the field, and has been called by Maxwell the principal founder of the science. We may regard his paper (1867), "Ueber die Art der Bewegung, welche wir Wärme nennen," as marking his definite entrance into this field, although many points were incidentally discussed in earlier papers.

This was soon followed by his papers, "Ueber die mittlere Länge der Wege, welche bei der Molecularbewegung gasförmiger Körper von den einzelnen Molecülen zurückgelegt werden," and "Ueber die Wärmeleitung gasförmiger Körper." A very valuable contribution to molecular science is the conception of the virial, defined in his paper (1870), "Ueber einen auf die Wärme anwendbaren Satz," where he shows that in any case of stationary motion the mean vis viva of the system is equal to its virial. In the mean time, Maxwell and Boltzmann had entered the field. Maxwell's first paper, "On the Motions and Collisions of perfectly elastic Spheres," was characterized by a new manner of proposing the problems of molecular science. Clausius was concerned with the mean values of various quantities which vary enormously in the smallest time or space which we can appreciate. Maxwell occupied himself with the relative frequency of the various values which these quantities have. In this he was followed by Boltzmann. In reading Clausius, we seem to be reading mechanics; in reading Maxwell, and in much of Boltzmann's most valuable work, we seem rather to be reading in the theory of probabilities. There is no doubt that the larger manner in which Maxwell and Boltzmann proposed the problems of molecular science enabled them in some cases to get a more satisfactory and complete answer, even for those questions which do not at first sight seem to require so broad a treatment.

Boltzmann's first work, however (1866), "Ueber die mechanische Bedeutung des zweiten Hauptsatzes der Wärmetheorie," was in a line in which no one had preceded him, although he was followed by