110 THE ATOMIC WEIGHTS. 



CALCIUM. 



For determining the atomic weight of calcium we liave sets of experi- 

 ments by Berzelius, Erdmann and INIarchand, and Dumas. Salvetat* 

 also has published an estimation, l)ut without the details necessary to 

 enable us to make use of his results. I also find a reference f to some 

 work of Marignac, which, however, seems to have been of but little im- 

 portance. The earlier work of Berzelius was very inexact as regards 

 calcium, and it is not until we come down to the year 1824 that we find 

 any material of decided value. 



The most important factor in our present discussion is the composi- 

 tion of calcium carbonate, as Avorked out by Dumas and by Erdmann 

 and Marchand. 



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

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

 impurities of the material were also determined, the correction for them 

 applied, and the weighings reduced to a vacuum standard. The per- 

 centage of lime came out as follows : 



56.12 

 56.04 

 56.06 



Mean, 56.073, =b .016 



About this same time Erdmann and Marchand § began their researches 

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

 of impurity, gave respectively 56.09 and 56.18 pe;.' 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, zt .007 



It was subsequently shown b}^ Berzelius that calcium carbonate pre- 

 pared by this method retains traces of water even at 200°, and that 



*Compt. Rend., 17, 318. 1S43. 



t See Oudenian's monograph, p. 51. 



tCompt. Rend., 14, 537. 1842. 



g Journ. fiir Prakt. Chem., 26, 472. 1842. 



