BORON AND FLUORINE. 35 



proceed as follows: After the first evaporation with hydrofluoric 

 acid ignite the residue gently; any possible residual particles of 

 sodium formate are hereby decomposed and changed to the 

 carbonate, which is then easily decomposed by a second evapora- 

 tion with hydrofluoric acid. A decided excess of hydrofluoric 

 acid is desirable in this transposition in order to produce the acid 

 sodium salt, which is much more soluble than the normal salt. 

 The acid salt crystallizes without water and is easily dried. 

 Upon further ignition hydrogen fluoride is expelled, which, on 

 account of its eagerness to combine with water, carries with it 

 the last traces of the latter. The drying of sodium fluoride 

 presents no particular difficulties, although it became necessary 

 to fuse the salt before the final weighing, for the normal salt 

 produced by the decomposition of the acid salt is a very porous 

 mass which was found to take up traces of moisture. 



In the fusion of sodium fluoride several precautions had to be 

 observed because this salt, in spite of its high melting-point 

 (cir. 980), is rather volatile and its fusion in a crucible, even for 

 ordinary quantitative purposes, must be carried out with care. 5 

 Even in the present case, where the salt was contained in a long- 

 necked bulb, an appreciable loss of sodium was observed when 

 the salt was fused in a current of air. During the fusion, there- 

 fore, the air current was interrupted. When the mass had cooled 

 somewhat, dry air was again allowed to pass through the bulb 

 while the upper part of the apparatus was ignited. The entire 

 process was then repeated. In this manner it was possible to 

 dry and fuse the salt without loss and to re-fuse it without appre- 

 ciable change in weight. The rather close concordance in the 

 values obtained for the atomic weight of fluorine (table 3) is 

 probably due, in no small measure, to the precautions observed 

 in the fusion of the sodium fluoride. 



The fused sodium fluoride was changed to the sulphate in a 

 manner quite similar to the corresponding treatment of sodium 

 chloride in experiment iv, the only difference being that in the 

 present case the reaction proceeded more slowly. Owing to the 

 fused condition of the sodium fluoride, traces of the latter may 

 escape transposition even if the sulphuric acid be in excess and 



5 Some quantitative experiments on the volatility of sodium fluoride, when heated 

 in a crucible, are due to Waldbott, J. Am. Chem. Soc., 16, 418 (1894). 



