36 THE ATOMIC WEIGHTS OF 



even after the expulsion of this excess. It was imperative, there- 

 fore, to allow ample time for this interaction; after the first 

 expulsion of the bulk of the hydrogen fluoride it was best to 

 digest the residue with dilute sulphuric acid for some time before 

 the final evaporation. This mode of procedure is particularly 

 advisable when larger quantities of fused sodium fluoride are to be 

 transposed. Finally the sodium sulphate was fused and weighed 

 as described before. In the evaporation of sodium fluoride with 

 sulphuric acid in experiment ix an accident occurred and the 

 weight of the sodium sulphate, therefore, could not be recorded. 

 The solution of the residual sodium sulphate in water was 

 neutral in reaction. In two experiments the solution was acidified 

 with hydrochloric acid and tested with hydrogen sulphide; no 

 precipitation of platinum sulphide was observed. 



EXPERIMENT VI: THE RATIO OF SODIUM TETRABORATE TO SODIUM SULPHATE. 



In this experiment anhydrous borax was changed directly to 

 the sulphate without the intervention of either formic or a haloid 

 acid. The general procedure calls for little additional description. 

 The "borax glass" was fused to constant weight. After solution 

 in water a moderate excess of pure dilute sulphuric acid was 

 added, the mixture digested for some time, and gradually con- 

 centrated at a gentle heat in a current of air until, finally, a tem- 

 perature of about 140 had been reached; this temperature was 

 maintained for 2 hours. The expulsion of boric acid proceeds 

 quite readily in the presence of sulphuric acid. Even after the 

 first evaporation with water following the first distillations with 

 methyl alcohol, the green flame of the boric ester could no longer 

 be obtained, which indicates that the residual quantity of any 

 boric acid must have been quite small. Nevertheless, two more 

 treatments with water, alternating with repeated evaporations 

 with alcohol, were resorted to. The reasons for the ease with 

 which boric acid is thus expelled in the presence of sulphuric 

 acid is to be sought, no doubt, in the peculiar properties of the 

 latter acid ; its dehydrating action is favorable to the formation of 

 the boric ester and its "strength" and high boiling-point prevent 

 even the slightest reversion of the reaction under the conditions. 



The result obtained from this experiment is of interest. In 

 the first place, the presence of silica in the borax would have 

 made itself felt in this analysis. It is a common impurity in 



