OF ARTS AND SCIENCES. 



171 



TABLE III. — Deviations. 



discussion too prolonged for this paper ; but the subject cannot be com- 

 pleted without a brief statement of its relation to vfell known systems 

 and theories of Chemistry. A few extracts will serve as a basis to 

 show wliat the perissad law is intended to contribute to the solution 

 of the all-important problem. 



In 1854, in a paper read before the American Academy on "The 

 Numerical Relation between the Atomic Weights," Professor Cooke 

 announced the character of the problem as follows : — 



'• Numerical relations between the atomic weights of the chemical 

 elements have been very frequently noticed by chemists. One of the 

 fullest expositions of these relations was that given by M. Dumas, of 

 Paris, before the British Association for the Advancement of Science, 

 at the meeting of 1851. This distinguished chemist at that time 

 pointed out the fact, that many of the elements might be grouped in 

 triads, in which the atomic weight of one was the arithmetical mean 

 of those of the other two. Thus the atomic weight of Bromine is the 

 mean between those of Chlorine and Iodine ; that of Selenium is the 

 mean between those' of Sulphur and Tellurium; and that of Sodium, 

 the mean between those of Lithium and Potassium. 



" M. Dumas also spoke of the remarkable analogies between the 



