Approximation of Atomic Weights to Whole Numbers. 311 



by the presence of a fixed plane in air vibrating with velo- 

 city U. The energy of this motion is, per unit of volume, 

 ^ptJ m 2 , or for a stratum of height y resting upon unit of area, 



If we equate the two expressions we get a superior limit to 

 the thickness of the stratum whose energy could be absorbed 

 in time t. We find thus 



or, if we take n = 27rx256, and as for air fi/p = fu!='lfi, 

 y=llt. 



Thus in 9 seconds the thickness of a stratum of shadow 

 could not reach 1 metre, and must, in fact, be very much 

 less. It would appear therefore that this effect may be 

 neglected in practice, unless it be in the case of an observer 

 extremely close to the water. 



XXVI. On the Tendency of 'the Atomic Weights to approximate 

 to Whole Numbers, By the Hon. R. J. Strutt, Fellow of 

 Trinity College, Cambridge *. 



IT is now generally agreed that Prout's law, that the 

 atomic weights of the other elements are exact multiples 

 of that of hydrogen, is decisively contradicted by experiment. 

 The case of chlorine, of atomic weight 35*455, is in itself 

 conclusive, so far as the law as stated above is concerned. 



But, as all who have interested themselves in the subject 

 have noiiced, there remains the fact that many of the atomic 

 weights approximate very closely to whole numbers, so much 

 so as to suggest strongly that some law of nature is in question, 

 as distinct from the action of chance. 



It is important, therefore, to weigh the evidence as carefully 

 as possible ; and in order to do this, it appeared desirable to 

 calculate the chance that the atomic weights should approxi- 

 mate to whole numbers so closely as they are observed to do, 

 supposing their values to have been determined in some 

 entirely haphazard manner. This is the problem which 

 I propose to discuss in the present paper. After having 

 given some thought to the question, my attention was drawn 

 to a discussion of it by Prof. J. W. Mallet, in his paper on 

 the atomic weight of aluminium (Phil. Trans, vol. clxxi. 

 p. 1003). But as his method of treatment differs considerablv 

 from that to which I had been led, and as some of his data 

 are now obsolete, it seems worth while to reconsider it. 

 * Communicated by the Author. 



