COPPER. 95 



And the percentages are : 



Water at 260. At j/o . Cu in Cryst. Salt. Cu in CuSO t . 



7 2 5.45 2 



8 35-970 25.446 39.740(260) 



9 36.067 25.445 39-799(370) 



25.448 



In this series the determinations of sulphuric acid gave essentially the 

 same results for all three samples of sulphate, although one was not 

 dehydrated, and the others were heated to 260 and 370 respectively. 

 Hence the loss of weight in dehydration at either temperature represents 

 water only, and does not involve partial decomposition of the sulphate. 

 Between 360 and 400 copper sulphate is at essentially constant weight, 

 but further experiments indicated that even at 400 it retained traces of 

 water, and possibly as much as .042 per cent. The last trace is not ex- 

 pelled until the salt itself begins to decompose. 



Richards also effected two syntheses of the sulphate directly from the 

 metal by dissolving the latter in nitric acid, then evaporating to dryness 

 with sulphuric acid, and heating to constant weight at 400. 



.67720 grm. Cu gave 1.7021 grm. CuSO 4 . 39-786 per cent. Cu. 



1.00613 " 2.5292 " 39.78i " 



If we include these percentages in a series with the data from analyses 

 4, 6, and 9, which gave percentages of 39.811, 39.799, and 39.799 respect- 

 ively of copper in sulphate dried at 360 and upwards, the mean becomes 



CuSO 4 : Cu : : 100 : 39.795, .0036 



Since even this result is presumably too low, the other figures from 

 sulphate dried at 250 must be rejected. Since Hampe's work on the 

 sulphate is affected by the same sources of error, and apparently to a 

 still greater extent, it need not be considered farther. As for Richards' 

 nine determinations of Cu in CuS0 4 .5H 2 0, we may take them as one 

 series giving a mean percentage of 25.451, .0011. This salt seems to 

 retain occluded water, for the percentage of copper in it leads to a value 

 for the atomic weight which is inconsistent with the best evidence, as 

 will be seen later. 



In the second and third series of Richards' experiments upon copper 

 sulphate, the sulphuric acid was estimated by a method, which gave 

 valuable results. After the copper had been electrolytically precipitated, 

 the acid which was set free was nearly neutralized by a weighed amount 

 of pure sodium carbonate, and the slight excess remaining was deter- 

 mined by titration. Thus the weight of sodium carbonate equivalent to 

 the copper was ascertained. The resulting solution of sodium sulphate 

 was then evaporated to dryness, and a new ratio, connecting that salt 

 with copper, was also determined. The cross ratio Na 2 C0 3 : Na 2 S0 4 has 



