252 



SCIENCE 



[N. S. Vol. XXXIX. No. 



absolute motion, even though we may be un- 

 able experimentally to determine the absolute 

 motion; the change of mass and of length 

 ■which arise in moving systems are then but 

 the natural consequences of the redistribution 

 of the lines of force issuing from the moving 

 charges; our concept of time and distance is 

 no longer in need of modification ; we have es- 

 sentially the original Lorentz point of view. 

 The theory of relativity then is merely a col- 

 lection of results interpreted on moving axes 

 (with local time) and abstracted from the 

 underlying ether; the fundamental postulate 

 M of the theory, that we can not detect abso- 

 lute motion, is a natural consequence of the 

 fact that the transformations between differ- 

 ent sets of moving axes (and times) form a 

 group. For instance, if two particles move in 

 different directions through the ether each is 

 actually shortened in the direction of motion, 

 but observers attached to the particles can ob- 

 serve no shortening because everything in the 

 system is similarly shortened. And moreover, 

 since the transformations above mentioned 

 form a group, each observer, abstracting from 

 any conception of the ether and experimentally 

 unaware of any shortening in his system, con- 

 cludes that the system of the other observer is 

 shortened in the direction of their relative 

 motion and by the amount appropriate thereto. 

 On the other hand, if we take the point of 

 view that what we can not directly observe 

 does not exist, if we take the theory of rela- 

 tivity as itself fundamental and banish the 

 ether, then we have no such physical or con- 

 ceptual basis upon which to explain the short- 

 ening, the alterations in mass, or the changes 

 in time, and we are forced to change our con- 

 cepts of mass, length and time; we are forced 

 to all those new ideas which the theory of 

 relativity brings in and which seem incon- 

 gruous or bizarre to many persons, and these 

 ideas assume a semblance of naturalness only 

 when our universe is interpreted as four-di- 

 mensional with space and time unified and in- 

 herently interrelated, in the manner adopted 

 by Minkowski or Wilson and Lewis or Mc- 

 Laren. Which of the two points of view we 

 adopt depends largely upon our turn of mind. 



There are philosophers who feel that we are 

 entirely free to construct for ourselves any 

 image of the physical universe which seems 

 most natural and easy ; they will probably hold 

 to the ether as long as possible. There are 

 others who feel that we should not intrude 

 into the image any ideas which represent 

 things not immediately subject to experiment; 

 they will declare for the principle of relativity 

 as fundamental and not as derived, just as 

 Walther Ritz declared against electric and 

 magnetic field-intensities E and H. 



The author knows all this and covers most 

 of it in different parts of his work, but seems 

 nowhere to collect it. The brief discussion of 

 the mass of light is too indefinite to convey 

 any useful impression to me. The attempt at 

 the end of the work to outline a further ex- 

 periment bearing on the theory is laudable in 

 itself and shows that the author has thought 

 deeply into his subject from other sides than 

 the logical. 



Edwin Bidwell Wilson 



Massachusetts Institute or Technology 



Osmotic Pressure. By Alexander Findlay. 



Longmans, Green and Co., New York. 



Cloth, 8vo. Pp. 84. Price $1.00. 



This book by Dr. Findlay is one of the series 

 of monographs on inorganic and physical 

 chemistry of which he is the editor. The pur- 

 pose of these monographs is " to place before 

 advanced students of chemistry, accounts of 

 certain sections of inorganic and physical 

 chemistry fuller and more extended in scope 

 than can be obtained in ordinary text-books." 

 The present monograph deals with semi- 

 permeable membranes and osmotic pressure, 6 

 pages; van't Hoff's theory of dilute solutions, 

 4 pages; direct determination of osmotic pres- 

 sure of concentrated solutions, 12 pages; dis- 

 cussion of the recent determinations of osmotic 

 pressure and of the van't Hoff theory, 4 pages ; 

 the general theory of ideal solutions, 10 pages; 

 discussion of the osmotic pressure of aqueous 

 solutions of cane sugar in the light of the 

 theory of ideal solutions, 13 pages; indirect 

 determinations of the osmotic pressure, 15 

 pages; views regarding the canse of osmosis 



