CALCIUM. Ill 



minute quantities of chloride are also held b}^ it. These sources of error 

 are, however, in opposite directions, since one would tend to diminish 

 and the other to increase the weight of residue. 



In the same paper there are also two direct estimations of carbonic 

 acid in pure Iceland spar, which correspond to the following percentages 

 of lime : 



56.00 



56.02 . 



Mean, 56.01, =h .007 



In a still later paper '''^ the same investigators give another series of 

 results based upon the ignition of Iceland spar. The impurities were 

 carefull}' estimated, and the percentages of lime are suitabh'- corrected : 



4.2134 grm. CaCO. gave 2.3594 grm. CaO. 55-997 per cent. 



15.13S5 " 8.4S10 " 56.022 " 



23.5503 " 13.IQ5S " 56.031 



23.6390 " 13-2456 " 56-032 



42.0295 " 23.5533 " 56.044 



49.7007 " 27.8536 " 56.042 



Mean, 56.028, ±: .0047 



Six vears later Erdmann and Marchandf published one more result 

 upon the ignition of calcium carbonate. They found that the compound 

 1)egan giving off carbon dioxide below the temperature at which their 

 previous samples had been dried, or about 200°, and that, on the other 

 hand, traces of the dioxide were retained by the lime after ignition. 

 These two errors do not compensate each other, since both tend to raise 

 tlie percentage of lime. In the one experiment now under consideration 

 these errors were accurately estimated, and the needful corrections were 

 appli'ed to the final result. The percentage of residual lime in this case 

 came out 55.998. This agrees tolerably well with the figures found in the 

 direct estimation of carbonic acid, and, if combined with those two. gives 

 a mean for all three of 56.006, ± .0043. 



Combining all these series, we get the following result : 



Dumas 56.073, =fc .016 



Erdmann and Marchand 56.006, ± .007 



Erdmann and Marchand 56.02S, ± .0047 



Erdmann and Marchand 56.006, d= .0043 



General mean 56.0198, dr .0029 



For reasons given above, this mean is probably vitiated by a slight 

 constant error, which makes the figure a trifle too high. 



* Journ. fur Prakt. Chem., 31, 269. 1S44. 

 •f Journ. fiir Prakt. Chem., 50, 237. 1850. 



