THE CONVERSION OF ARGENTIC SULPHATE INTO CHLORIDE. 81 



trated sulphuric acid. 1 The gas then passed through the stopcock (K) to 

 the delivery tube (Af). The ground-glass stopper at the top of the tower 

 (/) acted as a safety-valve when the stopcock (K} was closed. Pure dry 

 air might also be delivered at N by means of the stopcock (L), in order to 

 sweep out the excess of acid at the conclusion of the reaction. 



The quartz tube containing the argentic sulphate was supported by 

 means of hooks of hard glass in front of the delivery tube (N), as before, 

 without making a tight joint. 



In seven of the final experiments a condenser tube of quartz (O) was 

 placed over the exit end of the tube (M) in order to condense the sul- 

 phuric acid and retain any silver which might escape. As will be shown, 

 only very small traces of silver were found in the distillate. The conden- 

 sation of sulphuric acid in the narrow tubes on the end was prevented by 

 keeping them hot by means of a small fixed fish-tail burner. 



A slow current of hydrochloric acid was generated and the tube warmed 

 gently. The reaction took place readily and quietly, the only difficulty 

 being that the argentic chloride formed was inclined to creep over the walls 

 of the vessel. This was probably clue to the liberated sulphuric acid hav- 

 ing dissolved undecomposed argentic sulphate ; the acid sulphate was then 

 transported by surface tension and converted into chloride in another place. 

 In two of the experiments one of the narrow end tubes was completely 

 blocked up in this manner, making a successful completion of the experi- 

 ment impossible ; but in other cases the difficulty was avoided by starting 

 with the argentic sulphate in a band in the middle of the tube. 



The sulphuric acid must be evaporated at a temperature below its 

 boiling-point in order to avoid loss by the projection of small particles. 

 The tube was heated by a flame held in the hand, the heating being regu- 

 lated by watching the escaping fumes of acid and also the color of the 

 argentic chloride in the tube. It is well known that as the temperature 

 increases, silver chloride acquires a deeper and deeper yellow color; and 

 after acquiring the necessary experience, this change of color proved very 

 helpful in regulating the temperature. 



After from 2.5 to 4 hours no more fumes of sulphuric acid could be seen 

 issuing from the tube. The argentic chloride was then fused very slowly 

 and quietly and kept in the fused state for 20 minutes in a current of hydro- 

 chloric acid. The tube was gently agitated in order to expose a fresh 



J In one of the preliminary experiments the hydrochloric acid was not dried. The 

 sulphuric acid first formed absorbed considerable water, thus becoming diluted and 

 nearly filling the tube with liquid sulphuric acid in which a large part of the silver 

 sulphate dissolved. After heating for some time, the whole mass was solidified. 

 This gave a non-porous mixture of silver sulphate and chloride. The hydrochloric 

 acid had no appreciable further action until the mixture was fused, and then the 

 action became very vigorous. The sulphuric acid which was formed boiled, and 

 caused spattering and therefore danger of mechanical loss. 



