Page:Aether and Matter, 1900.djvu/29

with regard to relative motions alone, in special cases: in other cases the stagnant aether is the ultimate datum of reference. Kirchhoff's exposition of the Lagrangian formulation on the basis of the Action principle applied to systems of 'particles.' The Lagrangian scheme implies that the system is conservative as regards its energy, and involves that it is conservative (e.g. reversible, dynamically permanent) in other respects. Hertz's objection that rolling motions are not included. Physical concepts are abstract ideas cultivated under the gu idance of observation. If matter is aethereally constituted, all dynamics ultimately rests on that of the aether: mechanical Action principle in this way deduced. The approximate inclusion of actual conservative material systems under the ideal Action principle involves observation and experiment. Thermodynamics a branch of statics: its aim is the formulation of the available energy, on which alone mechanical effect depends. Physical explanation of law of uniformity of temperature: temperature not a dynamical concept. Mechanical analogies. Mechanics of permanent systems is independent of molecular dynamics: illustrated by theory of osmotic pressure. Energy not an ultimate concept. Vital activity not mechanical as regards its stimulus.