RICHARDS AND I'AIIKEU. — ATOMIC WEIGHT OP MAGNESIUM. 65 



gravity of magnesic chloride used for this computation was the value 

 2.177 deterruiued by Playfair and Joule. Weighing \va.s done by sub- 

 stitution, the object to be weighed being placed on the right-hand pan 

 and balanced by tare weights on the left. In general, the precautions 

 used in the recent work done in this Laboratoi-y upon co|)per, barium, 

 strontium, and zinc were adhered to with great care.* We are indebted 

 to the Cyrus M, Warren Fund of Harvard University for some of the 

 platinum ware used in the following work. 



The atomic weights used in this investigation were as follows : — 



O . . . 16.000 Ag. . . . 107.930 



CI . . . 35. -156 



Method of Work. 



The method of operating may be inferred from the description of the 

 apparatus. The platinum boat, after having been weighed within its 

 wei"-hing bottle, was filled with the double chloride of ammonium and 

 magnesium and placed in position in the ignition tube, resting upon a 

 sort of carriage of platinum foil. The weighing bottle was placed with 

 its stopper in appropriate position in the " bottling tube," as previously 

 described. A current of dry hydrochloric acid gas was then passed 

 through the apparatus and the ignition tube was heated by a suitable 

 arrangement of burners. At first the I'esidual moisture was driven off 

 by the heat and carried away by the stream of gas. When as much 

 water as possible was expelled in this manner, the heat was slowly 

 increased so that the ammonic chloride commenced to vaporize. It was 

 found that the sublimation commenced before the salt was freed from the 

 last traces of moisture, but an effort was always made, by the very 

 gradual increase of heat, to make this proportion of water as small as 

 possible ; and it is probable that the salt was practically anhydrous some 

 time before the last of the ammonic chloride was sublimed. When no 

 further evolution of ammonic chloride could be observed, the heat was 

 increased until the tube and boat were heated to redness, and the mag- 

 nesic chloride had fused into a clear, colorless limpid liquid. It requires 

 a very excellent piece of combustion tubing to stand the heat necessary 

 to fuse magnesic chloride, and a number of tubes were spoiled during the 

 course of the work. In the first series of determinations the boat was 

 allowed to cool while the current of hydrochloric acid gas was still pass- 



* Kichards, These Proceedings, XXVI. 240 ; XXVIII. 1 ; XXIX. 55 ; XXX. 

 369 ; XXXI. 158. 



VOL. XXXII. — -5 



