3.1 8 THE POPULAR SCIENCE MONTHLY. 



motions of all the planets. This energy, he says, is dissipated more 

 and more widely through endless space, and never has been, probably 

 never can be, restored to the sun, without acts as much beyond the 

 scope of human intelligence as a creation or annihilation of energy, or 

 of matter itself, would be. 



From the earliest dawn of intellectual life, a general theory of the 

 constitution of matter has been a fruitful subject of debate, and human 

 science and philosophy have ever been dashing their heads against 

 the intractable atoms. The eagerness of the discussion was the greater, 

 the more hopeless the solution. For every man who set up an hy- 

 pothesis upon the subject, there were half a dozen others to knock it 

 down ; until at last speculation, which bore no fruit, was suspended. A 

 lingering interest still hung around the question, whether matter was 

 not infinitely divisible, and the atomic philosophers were not chasing 

 a chimera. From every new decision on this single point there was 

 an appeal, and the foothold which the atoms had secured in chemistry 

 was gradually subsiding. Of a sudden, the atomic theory has gained 

 a new lease of life. But the hero of the new drama is not the atom, 

 but the molecule. In all the physical sciences, including astronomy, 

 the war has been carried home to the molecules ; and the intellectual 

 victories of this and the next generation will be on this narrow field. 

 From the outlying provinces of physics ; from the sun, the stars, and 

 the nebulae ; from the comets and meteors ; from the zodiacal light 

 and the aurora ; from the exquisitely tempered and mysterious ether 

 the forces of Nature have been moving in converging lines to this 

 common battle-ground, and some shouts of victory have already been 

 heard. In the long and memorable controversy between Newton and 

 Leibnitz, and their adherents, as to the true measure of force, it was 

 charged against the Newtonian rule that force was irrecoverably lost 

 whenever a collision occurred between hard, inelastic bodies. The 

 answer was, that Nature had anticipated the objection, and had avoided 

 this kind of matter. Inelastic bodies were yielding bodies, and the 

 force which had disappeared from the motion had done its work in 

 changing the shape. But, unless the body could recover its original 

 figure by elasticity, there was no potential energy, and foi*ce was an- 

 nihilated. It is now believed, and to a large extent demonstrated, 

 that the force, apparently lost, has been transformed into heat, elec- 

 tricity, or some other kind of molecular motion, of which the change 

 of shape is only the outward sign. The establishment on a firm foun- 

 dation of theory and experiment of the so-called conservation of en- 

 ergy, the child of the correlation of physical forces, is one of the first 

 fruits of molecular mechanics. 



It is no disparagement of this discovery, on which was concen- 

 trated the power of several minds, to call it an extension, though a 

 vast one, of Newton's law of inertia, of Leibnitz's vis viva, and of 

 Huyghens's and Bernoulli's conservation of living forces ; these older 



