180 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 54 



artificial carbonate. The extreme difficulty of preparing absolutely pure 

 compounds of calcium is well known.' 



In the earliest of the three papers by Erdmann and Marchand there is 

 also given a series of determinations of the ratio between calcium car- 

 bonate and sulphate. Pure Iceland spar was carefully converted into 

 calcium sulphate, and the gain in weight noted. One hundred parts 

 of spar gave of sulphate : 



136.07 



136.06 



136.02 



136.06 



Mean, 136.0525, ± .0071 



Hence Ca = 40.025. 



In 1843 the atomic weight of calcium was redetermined by Berzelius,' 

 who investigated the ratio between lime and calcium sulphate. The 

 calcium was first precipitated from a pure solution of nitrate by means 

 of ammonium carbonate, and the thoroughly washed precipitate was 

 dried and strongly ignited in order to obtain lime wholly free from ex- 

 traneous matter. This lime was then, with suitable precautions, treated 

 with sulphuric acid, and the resulting sulphate was weighed. Correction 

 was applied for the trace of solid impurity contained in the acid, but not 

 for the weighing in air. The figures in the last column represent the 

 percentage of weight gained by the lime upon conversion into sulphate : 



1.80425 grm. CaO gained 2.56735 grm. 142.295 



2.50400 



3.90000 



3.04250 



3.45900 



Mean, 142.3998, ± .0518 



Hence Ca = 40.227. 



The atomic weight of calcium has been several times computed from 

 analyses of the chloride. The earliest determination by Berzelius^ was 

 based upon this compound, and Marignac * also used it in some provisional 

 experiments, to which, however, he assigns little importance. They gave 

 values for Ca far in excess of the truth. Dumas ' also published a series 

 of determinations of more than questionable value. Supposedly pure 



^ See Stas, Oeuvres Completes, 3, 337. 



^Journ. prakt. Chem., 31, 263. Ann. Chem. Pharni., -16, 241. 



^ Poggend. .\nnalen, S, ISO. 



■» Oeuvres Completes, 1, 90. 



= Ann. Chim. Ph.vs. (3), 55, 129. 1S50. Ann. Chem. Phaim., 113, 34. 



