69 



Six years later Erclmann and Marchaiid* published one 

 more result upon the ignition of calcium carbonate. They 

 found that the compound began 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 the percentage of lime. In the one experiment 

 now under consideration these errors were accurately esti- 

 mated, and the needful corrections were applied to the final 

 result. The percentage of residual lime in this case came 

 out 55.098. This agrees tolerably well with the figures 

 found in the direct estimation of carbonic acid, and, if com- 

 bined 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, dz .016 



Erdmann and Marchand 56.006, zt: .007 



" " 56.028, dz .0047 



" " 56.006, dr .0043 



General mean 56.0198, rh .0029 



For reasons given above this mean is probably vitiated 

 by a slight constant error, which makes the figure a trifle 

 too high. 



In the earliest of three papers by Erdmann and JNIarchand 

 there is also given a series of determinations of the ratio 

 between calcium carbonate and sulphate. Pure Iceland 



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



