THE UNITY OF PHYSICAL SCIENCE 5 



of the early part of the nineteenth century, it will be at once admitted 

 that great progress has been made. In addition to noteworthy ad- 

 vances and improvements along the lines laid down by these mas- 

 ters, there have been developed the relatively new fields of elasticity, 

 electromagnetics, thermodynamics, and astrophysics; and there has 

 been discovered the widest of all generalizations in physical science, 

 the law of conservation of energy. Whereas it was easy a century 

 ago to conceive, as in gravitational astronomy, of action at a dis- 

 tance across empty space, the universe in the mean time has come 

 to appear more and more plethoric not only with "gross matter," 

 but with that most wonderful entity we call the ether. The astro- 

 nomers have shown us, in fact, that the number of molar systems in 

 the universe is enormously greater than was supposed possible a cen- 

 tury ago ; while the physicists have revealed to us molecular systems 

 rivaling our solar system and its Jovian and Saturnian subsystems, 

 and they have loaded down the ether with a burden of properties 

 and relationships which its usual tenuity seems scarcely fitted to 

 bear. Whereas, also, a century ago the tendency of thought, under 

 the stimulus of the remarkable developments of the elastic solid 

 theory of light and the fluid theories of electricity, was chiefly to- 

 wards an ether whose continuity would have pleased Anaxagoras, 

 the tendency to-day is chiefly towards an ether whose atomicity 

 would have pleased Democritus. 



On the whole, it must be said that the advances of the past cen- 

 tury, and especially those of the past half -century, have been mainly 

 along the lines of molecular physics. The epoch of Laplace was dis- 

 tinctly an epoch of molar physics; the epoch of to-day is distinctly an 

 epoch of molecular physics. Light, heat, electricity, and magnetism 

 have been definitely correlated as molecular and ethereal pheno- 

 mena; while the recently discovered X-rays and the wonders of 

 radioactivity, along with the "electrons," the "corpuscles" and the 

 "electrions " of current investigations, all point towards a molecular 

 constitution of the ether. Thermodynamics, likewise, large as it has 

 grown in recent decades, is essentially a development of the mole- 

 cular theory of gases. It would be too bold, perhaps, to assert that 

 the trend of accumulating knowledge is towards an atomic unity of 

 matter, but the day seems not far distant when there will be room 

 for a new Principia and for a treatise which will accomplish for 

 molecular systems what the Mecanique Celeste accomplished for the 

 solar system. 



One of the most important advances of recent decades is found 

 in the fixation of ideas with respect to the units of physical science, 

 and in the great improvements which have been wrought in metro- 

 logy by the "International Bureau of Weights and Measures." Our 

 standards of length, mass, and time are now fixed with a degree 



