XI. 



The Numerical Relation hetiveen the Atomic Weights, ivith some Thoughts on the Classifi- 

 cation of the Chemical Elements. 



By JOSIAH p. COOKE, Jr., A.M., 



ERVING PROFESSOR OF CHEMISTRY IN HARVARD nNIVERSITV. 



{Communicated February 28, 1854.) 



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 Advance- 

 ment 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 remark- 

 able analogies between the properties of the members of these triads, comparing them 

 with similar analogies observed in Organic Chemistry, and drew, as is well known, 

 from these facts arguments to support the hypothesis of the compound nature of many 

 of the now received elements. Similar views to those of Dumas have been advanced 

 by other chemists. 



The doctrine of triads is, however, as I hope to be able to show in the present me- 

 moir, a partial view of this subject, since these triads are only parts of series similar 

 in all respects to the series of homologues of Organic Chemistry, in which the differ- 

 ences between the atomic weights of the members is a multiple of some whole number. 

 All the elements may be classified into six series, in each of which this number is 



VOL. V. NEAV SERIES. 33 



