OP ARTS AND SCIENCES. 177 



XL 



CONTRIBUTIONS FROM THE CHEMICAL LABORATORY OF 

 HARVARD COLLEGE. — J. F. Cooke, Director. 



FURTHER INVESTIGATION ON THE ATOMIC WEIGHT 



OF COPPER. 



By Theodore William Richards. 



Presented by J. P. Cooke, June 15, 1887. 



In the last volume of these Proceedings there appeared a description 

 of a new determination of the atomic weight of copper, based upon 

 the precipitation of silver from a neutral solution of argentic nitrate 

 by pure metallic copper. In the course of some further experiments, 

 it became necessary to ignite a portion of the silver from determination 

 No. 5 of that series ; and it was found that two grams of silver lost 

 four tenths of a milligram by this process. It is thus evident that 150° 

 is not a temperature high enough to drive out all the water from the 

 silver, and hence the results before given are incorrect by a slight 

 amount. In this determination the weight of the silver was 3.39035 

 grams, after drying at 150°, hence its weight after ignition would have 

 been 3.38975 grams. The weight of copper taken was .9987 grams, 

 therefore the corrected atomic weight of copper would be 63.452, in- 

 stead of 63.437. 



Unfortunately, all the silver formed in the other determinations had 

 been employed in testing for the presence of copper ; hence it was 

 impossible to determine whether the other samples would lose water 

 on heating in a similar manner. It seemed, therefore, desirable to 

 make a new series, using samples of pure copper, prepared from the 

 ores of different localities. Should the result be the same in each 

 instance, we should have a very strong proof, not only that the copper 

 used in each case was pure, — because the different samples would 

 probably contain different impurities, — but also that the atomic weight 

 of copper is a perfectly constant quantity. 



The method used was exactly that of the previous paper, although 

 more difficulty was found in keeping the solution below zero for 

 vol. xxm. (n. s. xv.) 12 



