Septembee 30, 1904.] 



SCIENCE. 



419 



with a burden of properties and relation- 

 ships which its usual tenuity seems scarcely 

 fitted to bear. Whereas, also, a century ago 

 the tendency of thought, under the stim- 

 ulus of the remarkable developments of the 

 elastic solid theory of light and the fluid 

 theories of electricity, was chiefly towards 

 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 century, and espe- 

 cially 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 corre- 

 lated as molecular and ethereal phe- 

 nomena; while the recently discovered 

 X-rays and the wonders of radioactivity, 

 along with the 'electrons,' the 'corpuscles' 

 and the 'electrions' of current investiga- 

 tions, all point towards a molecular consti- 

 tution of the ether. Thermodynamics, 

 likewise, large as it has grown in recent 

 decades, is essentially a development of 

 the molecular theory of gases. It would be 

 too bold, perhaps, to assert that the trend 

 of accumulating knowledge is towards an 

 atomic imity 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 metrology by 

 the 'International Bureau of Weights and 

 Measures.' Our standards of length, mass 

 and time are now fixed with a degree of 



precision which leaves little to be desired 

 for the present, and the capital resources 

 of measurement and calculation are now 

 available to an extent never hitherto ap- 

 proached. 



It should be noted, however, that con- 

 fidence in the stability of our standards is 

 by no means comparable with the perfec- 

 tion of their current applications. Indeed, 

 we may raise with respect to them the ques- 

 tion so long mooted with regard to the mo- 

 tions of the members of the solar system, 

 namely, are they stable? Notwithstanding 

 the admirable precision of the intereom- 

 parisons of the prototype meters and proto- 

 type kilograms and the equally admirable 

 precision of Professor Michelson's deter- 

 mination of the length of the meter in 

 terms of wave lengths of cadmium light, 

 we can not affirm that these observed rela- 

 tions will hold indefinitely. Our inherited 

 notions of mass have been rather rudely 

 shaken, also, by the penetrating criticisms 

 of Mach, and it appears possible even that 

 the law of conservation of mass may need 

 modification in the light of pending re- 

 searches. But worst of all, our time unit, 

 the sidereal day, is so far from possess- 

 ing the element of constancy that we 

 may affirm with practical certainty that it 

 is secularly variable. Having realized, 

 through Professor Michelson's superb de- 

 termination just referred to, the cosmic 

 standard of length suggested by Maxwell 

 thirty years ago, we are now much more in 

 need of an equally trustworthy cosmic 

 standard of time. 



If the progress of physics during the 

 past century has been chiefly in the direc- 

 tion of atomic theory, the progress of chem- 

 istry has been still more so. Chemistry is, 

 in fact, the science of atoms and molecules 

 par excellence, a distinction it has main- 

 tained for well nigh a full century under 

 the dominance of the fruitful atomic and 

 molecular hypotheses of Dalton and of 



