390 



SCIENCE 



[N. S. Vol. XXXVIII. No. 977 



to be set vibrating by anything less than a 

 certain minimum stimulus ; and that is the 

 basis for the theory of quanta. 



Quantitative applications of Planck's 

 theory, to elucidate the otherwise shaky 

 stability of the astronomically constituted 

 atom, have been made; and the agreement 

 between results so calculated and those ob- 

 served, including a determination of series 

 of spectrum lines, is very remarkable. One 

 of the latest contributions to this subject is 

 a paper by Dr. Bohr in the Philosophical 

 Magazine for July this year. 



To show that I am not exaggerating the 

 modern tendency towards discontinuity, I 

 quote, from M. Poincare's "Dernieres 

 Pensees," a proposition which he an- 

 nounces in italics as representing a form 

 of Professor Planck's view of which he ap- 

 parently approves: 



A physical system is susceptible of a finite 

 number only of distinct conditions; it jumps from 

 one of these conditions to another without passing 

 through a continuous series of intermediate con- 

 ditions. 



Also this from Sir Joseph Larmor's pref- 

 ace to Poincare's "Science and Hypoth- 

 esis ' ' : 



Still more recently it has been found that the 

 good Bishop Berkeley's logical jibes against the 

 Newtonian ideas of fluxions and limiting ratios 

 can not be adequately appeased in the rigorous 

 mathematical conscience, until our apparent con- 

 tinuities are resolved mentally into discrete aggre- 

 gates which we only partially apprehend. The 

 irresistible impulse to atomize everything thus 

 proves to be not merely a disease of the physicist: 

 a deeper origin, in the nature of knowledge itself. 



One very valid excuse for this prevalent 

 attitude is the astonishing progress that has 

 been made in actually seeing or almost see- 

 ing the molecules, and studying their ar- 

 rangement and distribution. 



The laws of gases have been found to 

 apply to emulsions and to fine powders in 



suspension, of which the Brownian move- 

 ment has long been known. This move- 

 ment is caused by the orthodox molecular 

 bombardment, and its average amplitude 

 exactly represents the theoretical mean free 

 path calculated from the "molecular 

 weight" of the relatively gigantic par- 

 ticles. The behavior of these microscop- 

 ically visible masses corresponds closely 

 and quantitatively with what could be pre- 

 dicted for them as fearfully heavy atoms, 

 on the kinetic theory of gases; they may 

 indeed be said to constitute a gas with a 

 gram-molecule as high as 200,000 tons; 

 and, what is rather important as well as 

 interesting, they tend visibly to verify the 

 law of equipartition of energy even in so 

 extreme a case, when that law is properly 

 stated and applied. 



Still more remarkable, the application 

 of X-rays to display the arrangement of 

 molecules in crystals, and ultimately the 

 arrangement of atoms in molecules, as ini- 

 tiated by Professor Laue with Drs. Fried- 

 rich and Knipping, and continued by Pro- 

 fessor Bragg and his son and by Dr. Tut- 

 ton, constitute a series of researches of high 

 interest and promise. By this means many 

 of the theoretical anticipations of our coun- 

 tryman, Mr. William Barlow, and — work- 

 ing with him — Professor Pope, as well as 

 of those distinguished crystallographers 

 von Groth and von Fedorow, have been 

 confirmed in a striking way. These bril- 

 liant researches, which seem likely to con- 

 stitute a branch of physics in themselves, 

 and which are being continued by Messrs. 

 Moseley and C. G. Darwin, and by Mr. 

 Keene and others, may be called an apothe- 

 osis of the atomic theory of matter. 



One other controversial topic I shall 

 touch upon in the domain of physics, 

 though I shall touch upon it lightly, for it 

 is not a matter for easy reference as yet. 

 If the principle of relativity in an extreme 



