ATOMIC WEIGHTS 177 



CALCIUM. 



Much of the older work on the atomic weight of calcium, including the 

 earliest determinations b}^ Berzelius, may be disregarded as having no 

 present value. Baup's ^ analyses of organic salts of calcium are inter- 

 esting, but carry no weight now. They led to the value Ca = 39.98. As 

 for Salvetat's ' determination, that was merely given as a statement of 

 results, without such details as would make his work available for dis- 

 cussion. 



The largest factor in measuring the atomic weight of calcium, is the 

 composition of calcium carbonate, as determined by several investigators. 

 This will be considered first, and the determinations based upon calcium 

 sulphate and calcium chloride folloAv later. 



In 1842 Dumas ' made three ignitions of Iceland spar, and determined 

 the percentages of carbon dioxide driven of¥ and of lime remaining. The 

 impurities of the ^naterial were also determined, the correction for them 

 applied, and the weighings reduced to a vacuum standard. His figures 

 are as follows : 



49.916 grm. CaCOj gave 28.01G grm. CaO. 56.12 per cent. 



50.497 " 28.305 " 56.04 



G4.508 " 36.167 " 56.06 



Mean, 56.073, ± .016 



Hence Ca = 40.171. 



About this same time Erdmann and Marcliand * began their researches 

 upon the same subject. Two ignitions of spar, containing .04 per cent, 

 of impurity, gave resepectively 56.09 and 56.18 per cent, of residue; but 

 these results are not exact enough for us to consider further. Four other 

 results obtained with artificial calcium carbonate are more noteworthy. 

 The carbonate was precipitated from a solution of pure calcium chloride 

 by ammonium carbonate, was washed thoroughly with hot water, and 

 dried at a temperature of 180°. With this preparation the following 

 residues of lime were obtained : 



56.03 

 55.98 

 56.00 

 55.99 



Mean, 56.00, ±: .007 



Hence Ca = 40.005. 



^ Bui). Universelle des Sciences de Geneve, 39, 347. 1S42. 

 - Compt. Rend., 17, 318. 1843. 

 ^Compt. Rend., 14, 547. 1842. 

 •'Journ. prakt. Chem., 26, 472. 1S42. 



