6 PHYSICAL SCIENCE 



of precision which leaves little to be desired for the present; and 

 the capital resources of measurement and calculation are now avail- 

 able to an extent never hitherto approached. 



It should be noted, however, that confidence in the stability of 

 our standards is by no means comparable with the perfection of 

 their current applications. Indeed, we may raise with respect to 

 them the question so long mooted with regard to the motions of 

 the members of the solar system: namely, are they stable? Not- 

 withstanding the admirable precision of the intercomparisons of the 

 prototype meters and prototype kilograms and the equally admir- 

 able precision of Professor Michelson's determination of the length 

 of the meter in terms of wave-lengths of cadmium light, we cannot 

 affirm that these observed relations 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 researches. But worst of all, our time-unit, the 

 sidereal day, is so far from possessing the element of constancy that 

 we may affirm with practical certainty that it is secularly variable. 

 Having realized, through Professor Michelson's superb determination 

 just referred to, the cosmic standard of length suggested by Max- 

 well thirty years ago, we are npw 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 direction of atomic theory, the progress of chemistry has been 

 still more so. Chemistry is, in fact, the science of atoms and mole- 

 cules par excellence, a distinction it has maintained for well-nigh 

 a full century under the dominance of the fruitful atomic and mole- 

 cular hypotheses of Dalton and of Avogadro and Ampere, and under 

 the similarly fruitful laws of gases established by Dalton and Gay- 

 Lussac. Perhaps the most striking feature of this progress, in a 

 general way, is the gradual disappearance it has entailed of the 

 imaginary lines which have been long thought to separate the fields 

 of chemistry and physics. Through the remarkable discoveries of 

 Faraday the two fields have been found to overlap in actual electrical 

 contact. Through the wonderful revelations of spectrum analysis, 

 originating with Bunsen and Kirchhoff, they have been proved to 

 be very largely common ground. And through the broader generaliz- 

 ations inaugurated by Willard Gibbs, Helmholtz, and others, they are 

 now both somewhat in danger of being annexed as a sub-province 

 of rational mechanics. 



To one whose work has fallen more especially in the fields of pre- 

 cise astronomy, geodesy, or metrology, it might seem a just reproach 

 to chemistry that it is a science whose measurements and calcul- 

 ations demand, as a rule, no greater arithmetical resources than 



