Philosophical State of the Physical Sciences. 505 



interfere. The conservation of energy, in its extended signifi- 

 cation, promises to be, like its forerunners, a valuable guide to 

 discovery, especially in the dark places into which physical 

 science has now penetrated. The caution which Lagrange has 

 given in reference to similar mechanical principles, such as the 

 conservation of the motion of the centre of gravity, the conser- 

 vation of moments of rotation, the preservation of areas, and 

 the principle of least action, is not without its applicability to 

 the new generalization. Lagrange accepts them all as results 

 of the known laws of mechanics and not as the essence of the 

 laws of nature. The most that physical science can assert is 

 that it possesses no evidence of the destructibility of matter or 

 force. 



It is not pretended that the existence of atoms has been or 

 can be proved or disproved. Some chemists think that the 

 atomic theory is the life of chemistry ; others have abandoned 

 it. Its importance is lost in that of the molecular theory. And 

 what has this accomplished to justify its existence ? If we de- 

 fine the molecule of any substance as the smallest mass of that 

 substance which retains all its chemical properties, we can start 

 with the extensive generalization of Avogadro and Ampere, that 

 the same volume of every kind of matter in the state of vapour, 

 and under the same pressure and temperature, contains an equal 

 number of such molecules. The conception of matter as con- 

 sisting of parts, which are perpetually flying over their micro- 

 scopic orbits and producing by their fortuitous concourse all the 

 observed qualities of bodies, is as old as Lucretius. He saw the 

 magnified symbol of his hypothesis in the motes which chase 

 one another in the sunbeam. One of the Bernoullis thought 

 that the pressure of gases might be caused by the incessant im- 

 pact of these little masses on the vessel which held them. The 

 discovery that heat was a motion and not a substance, fore- 

 shadowed by Bacon, made probable by Rumford and Davy, and 

 rigidly proved by Mayer and Joule when they obtained its exact 

 mechanical equivalent, opened the way to the dynamical theory 

 of gases. Joule calculated the velocity of this promiscuous 

 artillery, rendered harmless by the minuteness of the missiles, 

 and found that the boasted guns of modern warfare could not 

 compete with it. Clausius consummated the kinetic theory of 

 gases by his powerful mathematics, and derived from it the ex- 

 perimental laws of Mariotte, Gay-Lussac, and Chasles. By the 

 assumption of data more or less plausible, several mathematicians 

 have succeeded in computing the sizes and the masses of the 

 molecules and some of the elements of their motion. It should 

 not be forgotten that mathematical analysis is only a rigid sys- 

 tem of logic, by which wrong premises conduct the more surely 



