112 THE ATOMIC WEIGHTS. 



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 caijefully converted into 

 calcium suli)hate, 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 



In 1848 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 i)recautions, 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 b}^ the lime upon conversion into sulphate: 



1.80425 gim. CaO gained 2.56735 grm. 142.295 



2.50400 " 3-57050 " 142.592 



3.90000 " 5-55140 " 142.343 



3.04250 " 4.32650 " 142.202 



3.45900 " 4.93140 " 142.567 



Mean, 142.3998, dz .0518 



Last of all we have the ratio between calcium chloride and silver, as 

 determined by Dumas, f Pure calcium chloride was first ignited in a 

 stream of dry hydrochloric acid, and the solution of this salt was after- 

 wards titrated with a silver solution in the usual way. The CaCl^ pro- 

 portional to 100 parts of Ag is gi.ven in a third column : 



We have now' four ratios to comj^ute from, as follows : 



(i.) Percentage CaO in CaCOg, 56.0198, =h .0029 



(2.) CaO : SO3 : : lOO : 142.3998, ± .0518 



(3.) CaCOj : CaSO^ : : loo : 136.0525, ± .0071 



(4.) Ag2 : CaCl^ : : lOO : 51.4554, ± -O230 



* Journ. fCir Prakt. Chetn., 31, 263. Ann. Cheni. Pharm., 46, 241. 

 fAnn. Chim. Phys. (3), 55, 129. 1859. Ann. Chenj. Pharm., 113, 34. 



