MAGNESIUM. 105 



The third series was made upon very pure material, so 

 that the corrections, although applied, were less influential. 

 The results were as follows : 



4.2913 grm. MgCOg gave 2.0436 grm. MgO. 47.622 per cent. 



27.S286 " 13-2539 " 47-627 



14.6192 " 6.9692 " 47-672 " 



18.3085 " 8.7237 « 47.648 



Mean, 47.642, ± .0077 



In a supplementary paper* by Scheerer, it was shown 

 that an important correction to the foregoing data had been 

 overlooked. Scheerer, re-examining the magnesites in ques- 

 tion, discovered in them traces of lime, which had escaped 

 notice in the original analyses. With this correction the 

 two magnesites in question exhibit the following mean 

 composition : 



CO2- 



MgO 

 CaO . 

 FeO . 



Correcting for lime and iron, b}'^ assigning each its share 

 of CO2, the Snarum magnesite gives as the true percentage 

 of magnesia in pure magnesium carbonate, the figure 47.624. 

 To this, without serious mistake, we may assign the weight 

 indicated by the probable error, ± .0037 ; the quantity pre- 

 viously deduced from the percentages of MgO given in the 

 uncorrected analyses. 



From the Frankenstein mineral, similarl}' corrected, the 

 final mean percentage of MgO in MgCOs becomes 47.028. 

 This, however, represents three series of analyses, whose 

 combined probable errors may be properly assigned to it. 

 The combination is as follows : 



* Ann. (1. Cliem. unci Pharm., no, 240. 



