28 THE ATOMIC WEIGHTS OF 



bulb during such an ignition is, of course, highly important. 

 The term "constant" weight, as used in the sequel, should be 

 taken to imply a variation of not more than 0.03 milligram, unless 

 otherwise stated. A much closer agreement was often observed, 

 but in view of the experimental error in weighing such instances 

 must have been largely accidental. 



A sample of "borax glass" 2 was now transferred to the bulb 

 and kept fused in a current of dry air, free from carbon dioxide, 

 until its weight was constant. Needless to say, the inner platinum 

 tube, after the introduction of borax, was not again removed 

 until the entire experiment had been completed. It was found 

 that one gram of vitreous borax must be kept fused in this manner 

 for about an hour before it is worth while to take its weight. It 

 was then re-fused for a period of from 20 to 40 minutes, depending 

 on the size of the sample. In experiment xi, for instance, the 

 vitreous borax (after the preliminary weighing) was kept fused 

 for fully 90 minutes before reweighing. It still contained 0.1 

 milligram of water; for, after another fusion lasting 40 minutes, 

 the weight decreased by this amount. Another fusion, again of 

 40 minutes' duration, produced no further change in weight. 

 The flame of a large grid-burner served as a source of heat; the 

 time given refers to actual fusion. It should be added that the 

 final weight of the fused borax could in every case be fixed with 

 great precision. Numerous weighings and refusions showed that 

 the weight remained quite constant after the water had once 

 been expelled. 



The anhydrous borax thus obtained was digested, at a gentle 

 heat and for many hours, with a few cubic centimeters of purest 

 water. The quantity of water, as a rule, was sufficient to com- 

 pletely dissolve the borax at 70. Solution could be facilitated 

 by gentle agitation, as the bulb at this stage of the analysis was 

 not connected with the combustion tube. A slight excess of 

 formic acid (see p. 11) was then added and the mixture very 

 slowly evaporated to dryness in a current of air. As the contents 

 of the bulb were concealed from view and the progress of the 

 reaction could not be directly observed, ample time had to be 

 allowed. The dry residue was next digested with a few cubic 



1 The preliminary dehydration of borax has been described in Chapter I. 



