96 THE ATOMIC WEIGHTS. 



alread}^ been utilized in a previous chapter. The results, ignoring the 

 weights of h3'drated copper sulphate, are as follows, with the experiments 

 numbered as before : 



Cu. Na^CO;. NuoSO^ 



4 77S86 1.2993 1. 741 1 



6 1.90973 3.1S62 4-2679 



7 733S0 1.22427 1.63994 



8 92344 1.54075 



9 1.47926 3-30658 



Hence, 



Ch : Na.^CO-i : : 1 00 : X. Cu : Na.,SOi : : TOO : X. 

 166.824 ^ 223.549 



166.840 223. 4S2 



166. S40 223.538 



166.S49 223.529 



Mean, 166.S38, zh .0035 Mean, 223.525, =h .009S 



In one more experiment the sulphuric acid was weighed as barium 

 sulphate, the latter being corrected for occluded salts. 3.1902 grni. 

 CuS0,.5H.,0 gave 2.9761 BaSO, ; hence CuS0,.5H,0 : BaSO, : : 100 : 

 • 93.289. The sulphate contained 25.448 per cent, of Cu ; hence BaSO^: 

 Cu : : 93.289: 25.448. Still other ratios can be deduced from Richards' 

 work on the sulphate, but in view of the uncertainties relative to the 

 water in the salt the}^ are hardly worth computing. 



In his third paper upon the atomic weight of copper,* Richards studied 

 the dibromide, CuBr^, \\\ preparing this salt he used hydrobromic 

 acid made from pure materials, and farther purified by ten distillations. 

 This was saturated with copper oxide prepared from |)ure electrol.ytic 

 copper, and the solution obtained was proved to be free from basic salts. 

 As the crystallized compound was not easily obtained in a satisfactory 

 condition, weighed quantities of the solution were taken for analj^sis, in 

 which, after expulsion of bromine by nitric and sulphuric acids, the 

 copper was determined by electrolysis. In other portions of solution 

 the bromine was precipitated by silver nitrate, and weighed as silver 

 bromide. The first preliminary series of experiments gave the subjoined 

 results, with vacuum weights as usual : 



/)i 25 Grdiiiines of Solution. 



Cu. AgBr. 



.4164 2.4599 



.4164 2.4605 



.4164 2.4605 



.4165 2.4599 



Hence 2 AgBr : Cu : : 100 : 16.927, ± .0013. 



* Proc. Amer. Acad., 25, 195. 1890. 



