174 PROCEEDINGS OF THE AMERICAN ACADEMY. 



had passed through it, one may safely assume that the purification was 

 sufficient. The gas was dried by means of sulphuric acid and phos- 

 phoric peutoxide. 



Method of A nalysis. — Perhaps the best method of explaining the 

 method of analysis is to give a detailed description of a single deter- 

 mination ; and for this purpose Analyses 15 and 19, in which both 

 silver and argentic bromide were weighed, will best serve. 



The very pure sublimed zincic bromide was pressed into a platinum 

 boat ; and the boat was placed in a tube of hard glass, which had been 

 ground into another tube designed to contain a weighing bottle. The 

 apparatus consisted essentially of a combination of the two pieces of 

 apparatus shown upon pages 167 and 1G8; it was devised for a 

 research upon the atomic weight of magnesium now being carried on 

 by Messrs. Richards and Parker, and it will be described in full when 

 that investigation is pixblished. With the help of this apparatus 

 it was possible to heat the zincic bromide to any temperature below 

 its boiling point in an atmosphere of pure dry air, pure dry carbon 

 dioxide, or pure dry carbon dioxide charged with hydrobromic acid ; 

 and these gases could be changed at will merely by the opening and 

 closing of stopcocks. When the heating had been continued for the 

 desired length of time, it was possible to push the boat into the weigh- 

 ing bottle and to stopper the weighing bottle very tightly in a perfectly 

 dry atmosphere, without the least chance of the absorption of moigture 

 from the outside air. All the apparatus which could possibly come 

 into contact with bromine or hydrobromic acid was made of glass, 

 with ground glass joints and glass gridirons for convenient refilling. 



The zincic bromide was heated very gradually at first in an atmos- 

 phere of carbon dioxide which had been dried by passing over sul- 

 phuric acid, fused zincic bromide, and phosphorus pentoxide. If 

 heated very gradually in this way, zincic bromide may he almost 

 wholly dehydrated without loss of bromine ; but a basic bromide is 

 certain to form if the heating is rapid. When all of the apparent 

 water had been expelled from the substance and its containing tube, 

 dry hydiic bromide was added to the carbonic dioxide, and the tem- 

 perature was gradually raised to the fusing point of zmcic bromide. 

 The bromide was kept at a temperature just above its melting point 

 for about an hour ; during this time perhaps a tenth of the substance 

 sublimed in the exit end of the " combustion " tube, — rendering the 

 drying tube — which had been ground on to protect the exit — un- 

 necessary. It was assumed that at the end of an hour the fused zincic 

 bromide must be as free from water and from basic salt as it was 



