The Numerical Relation between the Atomic Weights. 387 
Arr. XLIL.—-The Numerical Relation between the Atomic 
Weights, with some Thoughts on the Classification of the 
Chemical Elements; by Jostau P. Cooke, Jr., A.M., Erving 
Professor of Chemistry in Harvard University.* 
Noumericat relations between the atomic weights of the chem- 
ical 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 ele- 
ments 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 bromiuc is the mean between those of chlo- 
tine and iodine; that of seleninm 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 prorerties of the members of 
these triads, comparing them with similar analogies observed in 
organic chemistry, aud 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 o 
to characterize its series. In the first it is nine, in the second 
eight, in the third six, in the fourth five, in the fifth four, and in 
the last three. The discovery of this simple numerical relation, 
Which includes all others that have ever been noticed, was the 
sult of a classification of the chemical elements made for the 
Purpose of exhibiting their analogies in the lecture-room. A 
Short notice of this classification will, therefore, make a natural 
Introduction to the subject. 
very teacher of chemistry must have felt the want of some 
system of classification like those which so greatly facilitate the 
acquisition of the natural-history sciences. In most elementary 
text-books on chemistry, the elements are grouped together with 
little regard to their analogies. Oxygen, hydrogen, and nitrogen 
are usually placed first, aud therefore together, although there are 
ly to be found three elements more dissimilar ; again, phos- 
* Communicated to the American Academy, Boston, Feb. 28, 1854. 
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