AQUEOUS AND ALCOHOLIC SOLUTIONS. 153 



ence between the two cases. The experiment with tellurium is therefore, I con- 

 ceive, to be explained simply by supposing that, in the presence of a readily re- 

 ducible metallic oxide, the nascent hydrogen rather combines with the oxygen of 

 that oxide than with tellurium. 



In DAVY'S Bakerian Lecture of 1807, he states, that in one experiment, 

 where nitrate of silver was made positive, and distilled water negative, the asbes- 

 tos Avhich formed the connection was found to be coated with metallic silver. I 

 do not doubt the result ; but I conceive that, under the powerful voltaic action 

 employed (apparently 100 pairs of 4 or 6 inch plates), the oxide of silver carried 

 over to the negative side had suffered reduction either by the simple action of 

 light, or more probably by hydrogen evolved in the negative tube. I made the 

 same experiment, using only fifty pairs of 2-inch plates, and found that neither 

 silver nor oxide appeared on the asbestos in forty minutes, but, on reversal, the 

 negative foil was found fringed with metallic silver in half an hour. A strong 

 solution of nitrate of silver was also placed in a bent tube, and distilled water 

 poured above it. The negative foil was then plunged in the distilled water, and 

 the positive in the solution, but no oxide nor silver appeared anywhere in half an 

 hour, whilst, on reversal, metallic silver began to be formed on the negative foil 

 in ten minutes. What, then, is the difference between the two positions ? Simply 

 this, that, in the second, nascent hydrogen is evolved in the solution itself. 



In addition to the proof formerly adduced* of the secondary origin of the 

 electro-negative constituent of the haloid salts in their aqueous solutions, I may 

 mention a very simple experiment which leads to the same conclusion, and from 

 being capable of extension, as shall be afterwards shewn, to alcoholic solutions of 

 those salts, throws light on their nature also. If an aqueous solution of iodide of 

 potassium is acted on, using platinum poles, iodine immediately separates at the 

 positive pole, and is dissolved by the liquid giving it a deep red colour. But if, 

 instead of platinum, a positive pole of zinc is employed, the liquid being in the bent 

 tube A, and the power fifty pairs of 2-inch plates, there is not the slightest 



discoloration of the liquid ; but a speedy and copious deposition of oxide of zinc 

 takes place from the positive pole, with only a bubble or two of elastic fluid, whilst 

 at the negative pole there is brisk effervescence. These appearances are inexpli- 

 cable, unless on the idea of the secondary origin of the iodine ; and they are best 

 explained by assuming that the oxygen from decomposed water combines with 



* Edin. Trans, xiii. 344, and xiv. 118. 



