BORON AND FLUORINE. 31 



In order to insure a complete interaction the residue, after the 

 expulsion of the hydrogen chloride, was dissolved in a little water 

 and concentrated by evaporation. Finally the temperature was 

 gradually raised to expel the excess of the acid. The last traces 

 of sulphuric acid were removed by keeping the sodium sulphate 

 in a state of fusion for some time. The evaporations were acceler- 

 ated by a current of air. When the sodium sulphate had been 

 kept fused for about 30 minutes the bulb was lowered so as to 

 bring the lower (curved) end of the inner tube within the neck 

 of the bulb. This part of the inner tube could thus be raised to a 

 much higher temperature and the complete removal of the acid 

 and decomposition of possible traces of bisulphate be made certain. 

 These operations were then repeated. It was found that after 

 the free sulphuric acid had once been expelled in this manner, 

 the weight of the normal salt remained remarkably constant, 

 even after prolonged fusion in the jacketed bulb, with a current 

 of air passing through the latter. 



After the completion of the experiment the sodium sulphate 

 was dissolved in a little water. The solution was neutral to 

 methyl orange; the addition of a trace of dilute hydrochloric acid 

 produced a pink tint. In converting the sodium chloride into 

 the sulphate an excess of sulphuric acid was not to be avoided 

 and the complete removal of this excess proved rather laborious. 

 Nevertheless the results and the test show that the method was 

 entirely reliable. 



The sulphuric acid used in this experiment, and in those to 

 follow, was prepared by distilling an excellent "C. P." grade from 

 a long-necked Jena retort. Only the middle portion of the dis- 

 tillate was collected and redistilled as before. The acid was 

 diluted and used as indicated. 



EXPERIMENTS V, IX, X, XI : THE RATIOS BETWEEN SODIUM TETRABORATE, SODIUM 



FLUORIDE, AND SODIUM SULPHATE. 



The method finally adopted to convert borax into sodium 

 fluoride has been indicated in Chapter I. That the direct con- 

 version of the borate into the corresponding fluoride by means 

 of hydrofluoric acid and methyl alcohol was most unpromising 

 has also been emphasized. There was still another method which 

 seemed to be entitled to a fair trial. This consisted in igniting 

 a mixture of powdered borax and ammonium fluoride, whereby 



