CONTRIBUTIONS FROM THE CHEMICAL LABORATORY OF 

 HARVARD COLLEGE. 



A NEW INVESTIGATION CONCERNING TPIE ATOMIC 

 WEIGHT OF URANIUM.* 



By Theodore William Richards and Benjamin Shores Merigold. 



Presented December 11, 1901. Received December 19, 1901. 



Introduction. 



Our knowledge of uranium dates from the year 1789, when it was first 

 recognized as an element by Klaproth. It can by no means, therefore, 

 be classed with the new elements, nor is it of great rarity. Nevertheless, 

 comparatively few determinations of the atomic weight of this element 

 have been made, and of these, one only has been carried out with the 

 degree of accuracy necessary in work of this kind. During the fifty 

 years following the discovery of uranium a number of atomic weight 

 determinations were made by Berzelius, Arfvedson, Schonberg, Mar- 

 chand, and Rammelsberg. This early work is now of historical interest 

 only, for the results vary widely, and in some cases are of such a nature 

 as scarcely to be considered quantitative, in the modern sense of the 

 word. For example, Rammelsberg obtained results varying from 184 

 to 234, calculated upon the modern basis. 



In 1841 Peligot discovered that the substance then known as uranium 

 was not an element, but an oxide. This discovery, while it did not 

 impair the value of the analytical work previously done, necessitated a 

 recalculation of the numerical value of the atomic weight. The new 

 value was 120, and this remained practically unchanged during the next 

 thirty years. When the periodic classification of the elements was first 

 suggested, uranium, with the atomic weight 120, was one of the elements 

 for which there was no place. From a study of the properties of 

 uranium and its compounds, Mendeleefi" declared that the atomic weight 



* The greater part of the work described in this paper was presented to the 

 Faculty of Arts and Sciences of Harvard University by B. S. Merigold, as a thesis 

 for the degree of Ph.D., in June, 1901. 



