140 THE ATOMIC WEIGHTS. 



magnesium carbonate, the 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 MgCOj becomes 47.628. This, however, represents 

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

 erly assigned to it. The combination is as follows : 



zfc .0022 

 ± .0069 

 rb .0077 



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



^Ve may now combine tlie results obtained from l)oth magnesites: 



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



Frankenstein mineral " 47.628, zfc .0020 



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



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 Aveighing, 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 

 proportional to 100 parts of silver, the quantities of MgCl^ stated in the 

 third column : 



2.203 g''™- MgClj ^ 4.964 grm. Ag. 44.380 



2.5215 " 5.678 " 44.408 



2.363 " 5-325 " 44-376 



3,994 " 9.012 " 44.319 



2.578 " 5.834 " 44-189 



2.872 " 6.502 " 44.171 



2.080 " 4-7IO " 44.161 



2.214 " 5-002 " 44.262 



2.086 " 4.722 " 44.176 



1. 688 " 3.823 " 44.154 



1.342 " 3.031 " 44.276 ■ 



Mean, 44.261, rfc .020 



Tliis determination gives a very high value to the atomic weight of 

 magnesium, which is unquestionably wrong. The error, jn-obably, is 

 due to the presence of oxychloride in the magnesium chloride taken, an 



*Ann. Chem. Pharm., 113, 33. i860. 



