352 Mr. Adie's Experiments with Galvanic Couples 



of iron, and treating the filtered liquid with hydro sulphuret 

 of ammonia, which gave a yellowish white precipitate. This 

 result proves that the minute transparent crystals among the 

 fibres of the thread contained protoxide of zinc. 



On a former occasion I employed either the air-pump or 

 ebullition to deprive water used in exciting voltaic couples, of 

 absorbed air, I gave preference to the method of boiling the 

 water in the battery cell, as the more severe test, for showing 

 how far a battery's action depended on oxygen from the atmo- 

 sphere. The proof which appeared to me to furnish satisfac- 

 tory evidence of the assistance given by absorbed oxygen, was 

 when the indication in the galvanometer fell near to zero by 

 prolonged boiling, and rose again when water holding dis- 

 solved air was thrown into the cell. According to this test, 

 a zinc and platinum couple lose much of their action when 

 excited by pure water boiled for near two hours. The galva- 

 nometer needles always indicated a slight action, however long 

 the boiling was prolonged ; but as 1 found when care was 

 taken to have an atmosphere of steam resting on the surface 

 of the boiling water the action of the couple was at its lowest, 

 I was led to think that what remained might be due to oxy- 

 gen from the atmosphere, which it was impossible to remove 

 perfectly. The experiment given above renders this view no 

 longer tenable ; for if zinc and copper elements can at ordinary 

 temperatures slowly generate gas, it must follow that all the 

 elements less oxidizable than copper will at boiling tempera- 

 tures possess, when associated with zinc, a voltaic action 

 independent of oxygen from the atmosphere. 



To try the effect of a zinc and copper couple "excited by 

 pure boiling M^ater, I attached a pair of plates to a more sensi- 

 tive galvanometer than I had hitherto used : the plates were 

 placed in a Florence flask and covered to a depth of two 

 inches with pure water previously distilled in glass vessels ; 

 there was only a small orifice in the cork at the top of the 

 flask for a steam escape, in order to preserve the boiling sur- 

 face from the atmosphere. 



Previous to boiling, the galvanometer needle stood at 50° 



Indication the moment boiling was about to begin , 70° 

 after long boiling 20^^ 



A similar experiment with iron and copper elements : — 



Indication previous to boiling 20° 



at boiling 46° 



after long boiling 7° 



In this experiment the indication rose on cooling to 30°, 

 and afterwards fell back. 



When silver or platinum was substituted for the copper the 



