ATOMIC WEIGHTS. 



529 



it follow that there is no truth in the law. Far from it ; at 

 the end of his paper (4) on the atomic weight of aluminium, 

 Professor Mallet gives a list of the atomic weights of eigh- 

 teen elements which were then most accurately known, and 

 points out that of these " ten approximate to integers within a 

 range of one-tenth of a unit. What then is the degree of pro- 

 bability that this is purely accidental, as those hold who carry 

 to the extreme the conclusions of B E RZ E L I U S and ST AS ? 

 . . . The probability in question is found to be only equal to 

 1 : I097"8." In the last sentence of the paper Professor 

 Mallet remarks "that not only is Prout's Law not as yet 

 absolutely overturned, but that a heavy and apparently in- 

 creasing weight of probability in its favour, or in favour of 

 some modification of it, exists and demands consideration ". 

 Another point, indicating how little we know as to the 

 true meaning of striking numerical relationships, is, that 

 when we have three members of the same group of ele- 

 ments the intermediate atomic weight is always a little less 

 than the arithmetical mean of the other two. This is clearly 

 shown by the following- examples. The atomic weights 

 taken are referred to the standard O = 16. 



Lithium — 

 Sodium = 

 Potassium = 



Sulphur = 

 Selenium = 

 Tellurium = 



7-030 



23"°5 8 

 39 -1 36 



32-06 



mean 

 23"°83 



Magnesium = 24-38 



mean 

 79-o7 J 79-83 

 127-60 ) 



Calcium = 

 Strontium = 

 Barium = 



Zinc 

 Cadmium 



Chlorine 

 Bromine 

 Iodine 



mean 

 68-2 



= 65-38 



— II2'o8 



= 35453 



= 79-963 r8r 158 

 = 126-864; 



mean 



4C00 ) 



^ / mean 



8 7'5 2 [88-72 



137-44 ) 



The consideration of such groups leads us naturally to 

 another point into which we cannot enter here, viz. : do the 

 properties of the elements vary in any definite way with their 

 atomic weights ? and the answer to this is given by the state- 

 ment of the Periodic Law as developed by Newlands, 

 Lothar Meyer and Mendelejeff, which states in its simplest 

 form that " the properties of the elements form a periodic 

 function of the atomic weights of the elements ". This law 

 has received very general acceptance in spite of two or three 



