A REVISION OF THE ATOMIC WEIGHT OF ARSENIC. 79 



not be completely removed even by keeping the silver chloride fused in the cur- 

 rent of hydrochloric acid for as long as 8 hours. This is the cause of the larger 

 quantities of arsenic found in the chloride obtained in the earlier analyses. 

 Furthermore, the longer period of heating at a temperature above the fusing 

 point of silver chloride accounts for the larger amounts of volatilized silver 

 chloride found in these experiments. 



As experience was gained, it was found best to expose the salt first in the cold 

 for about 8 hours to the action of the hydrochloric-acid gas, next to heat the 

 salt gently below its fusing point for from lo to 15 hours, and finally to keep it 

 barely fused for from 5 to 10 hours longer. When the reaction was apparently 

 at an end, the current of hydrochloric-acid gas was stopped, and dry air was 

 passed through the tube for about 15 minutes in order to eliminate hydro- 

 chloric acid. The silver chloride was then allowed to sohdify in a uniform thin 

 layer around the inside of the quartz tube by slowly revolving the tube during 

 solidification. The platinum wire used in weighing the tube was shpped on, the 

 tube was transferred to its desiccator, and after standing several hours beside 

 the balance it was weighed. 



In order to make sure that the reaction was complete the silver chloride was 

 again fused, and exposed to the action of hydrochloric acid for several hours 

 longer. As a rule, no change in weight was observed. In all cases constant 

 weight was obtained upon heating in the same way for a third time. 



After making certain that only a small quantity of arsenic, if any, remained 

 in the silver chloride, the contents of the quartz tube were dissolved in ammonia, 

 and the silver chloride was reprecipitated by boiling the solution to expel the 

 ammonia and adding a small quantity of sulphuric acid. The solution, after 

 evaporation, was added to a Berzelius-Marsh apparatus containing arsenic-free 

 zinc and sulphuric acid, and a mirror of arsenic was deposited in a hard-glass 

 capillary tube in the usual way. The hydrogen was dried by calcium chloride 

 before passing into the hard-glass tube, and the generating flask was cooled with 

 water to prevent the evolution of hydrogen sulphide. 



The arsenic mirror formed was compared with a photograph of standard 

 arsenic mirrors,^ the original mirrors showing that comparison with the photo- 

 graph was equally satisfactory. The correction was applied by assuming that 

 the arsenic was present in the silver chloride as arsenic trichloride, although 

 if present as silver arsenate the correction would be much smaller. In any 

 case the correction for residual arsenic is so small as to be almost without effect 

 upon the final result. 



Ebaugh used essentially the same method of heating the arsenate in hydro- 

 chloric acid, although the periods were shorter, so that it is probable that the 

 small quantities of arsenate used (scarcely over one gram in any analysis) did 

 not retain weighable amounts of arsenic. 



1 Sanger: Proc. Amer. Acad., 26, 24 (1891). 



