270 PROCEEDINGS OP THE AMERICAN ACADEMY 



determination must be made the subject of an especial investigation ; 

 and for this, with so remote a purpose as the present one, time was 

 wanting. 



TJie Atomic Weight of Copper. 



The calculation of the atomic weight of copper from the amount of 

 metal contained in cupric sulphate which has been dried at 250-270° 

 gives the value 63.35. This result sufficiently approaches Hampe's 

 (63.32) to show the identity of the materials used. But it has been 

 proved that such cupric sulphate still contains a comparatively large 

 amount of water ; and this fact renders useless the careful analyses 

 made in 1874. The effect of the coi-rection is considered in a follow- 

 ing paragraph. 



Besides this method of calculation " from difference," the data 

 which have been given afford twelve other ratios for the computation 

 of the atomic weight of copper. Six of these ratios are useless be- 

 cause of the known error in the crystal water ; namely, 5 H.,0 : Cu ; — 

 SO4 . 5 H2O : Cu ; — 5 H^O : CuSO^ ; — NaoCOa : CuSO^ . HoO ; — 

 NasSOi : CUSO4 . 5 H2O ; — and BaSOi = CuSOi . 5 H2O. Three of 

 these last ratios give values which are much too low, and the other 

 three give values which are much too high, as would naturally be 

 expected. The mean is curiously near to the true value, but need not 

 be further discussed. Neglecting these results because of the known 

 constant error which vitiates them, there still exist seven ratios which 

 do not involve the uncertain amount of the water of crystallization. 



I. From calculation based upon the results tabulated on page 267 

 anhydi'ous cupric sulphate is seen to contain certainly over 39.807 

 per cent of copper, and very probably as much as 39.832 per cent. 

 The first number gives 63.53 as the atomic weight of copper, and this 

 value must be regarded as the lowest possible limit. The second 

 much more probable figure, which takes into account the water held 

 by cupric sulphate at 400°, gives the jDroportion : 



(CUSO4 - Cu) : Cu = (100.000 - 39.832) : 39.832 = 96.06 : 63.593. 



II. From Experiments 4, 6, 7, and 8 it is found that 7.2501 grams 

 of sodic carbonate correspond to 4.34583 grams of copper when 

 methyl orange is used as an indicator ; and from Experiments 6, 7, 

 and 8 it is found that 5.95165 grams of sodic carbonate correspond 

 to 3.56697 grams of copper when phenol phtlialcin is used as an indi- 

 cator. It has been shown that the impurities of the sodic carbonate 



