ATOMIC WEIGHTS 217 



examining the magnesites in question, 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 : 



Snarum. Frankenstein. 



CO2 52.131 52.338 



MgO 46.663 47.437 



CaO 430 .225 



FeO 776 



100.000 100.000 



Correcting for lime and iron, by assigning each its sliare of CO,, the 

 Snarum magnesite gives as the true percentage of magnesia in pure 

 magnesium carbonate, tlie figure 47.624. To this, without serious mis- 

 take, we may assign the weight indicated by the probable error. ±.0037, 

 the quantity previously deduced from the percentages of MgO given in 

 the uncorrected analyses. 



From the Frankenstein mineral, similarly corrected, the final mean 

 percentage of MgO in MgCOg becomes 47.628. This, however, represents 

 three series of analyses, whose combined probable errors may be prop- 

 erly assigned to it. Tlie combination is as follows : 



± .0022 

 ± .0069 



±: .0077 



Result, ± .0020, probable error of the general mean. 



We may now combine the results obtained from both magnesites : 



Snarum mineral Per cent. MgO, 47.624, ± .0037 



Frankenstein mineral " 47.628, it .0020 



General mean Per cent. MgO, 47.627, ± .0018 



Hence Mg = 24.016. 



The next investigation upon the atomic weight of magnesium which 

 we have to consider is that of Dumas." Pure magnesium chloride was 

 placed in a boat of platinum, and ignited in a stream of dry hydrochloric 

 acid gas. The excess of the latter having been expelled by a current of 

 dry carbon dioxide, the platinum boat, still warm, was placed in a closed 

 vessel and weighed therein. After weighing, the chloride was dissolved 

 and titrated in the usual manner with a solution containing a known 

 quantity of pure silver. The weighings which Dumas reports give, as 

 poportional to 100 parts of silver, the quantities of MgCL stated in the 

 third column : 



1 .\nn. Chem. Pharm., 113, 33. 1S60. 



