254 



either method was observed on varying the strength of the electric 

 current, or on varying either the actual or relative distances of the 

 electrodes from the metallic globule. 



45. The presence of an electro-positive metal in one portion of 

 the surface of the mercury will (by generating a small electric 

 current) sometimes cause rotation of the electrolyte after the battery- 

 wires are removed, especially if the mercury is touched with a pla- 

 tinum wire beneath the surface of the liquid ; this is seen most 

 frequently with mercury into which some alkali-metal has been 

 deposited. 



46. The general phenomena of .the movements may be briefly 

 redescribed thus : A. When both the wires are in the electrolyte, 

 and the mercury between them, several cases occur : 1 . With a 

 strongly alkaline liquid, a positive flow of the solution from the 

 positive wire over the mercury to the negative wire occurs ; 2. With 

 a strongly acid liquid, a negative flow of the solution takes place ; 

 and 3. With a solution of a neutral or slightly alkaline salt, espe- 

 cially of a salt composed of a strong acid and a strong base, two 

 flows occur, a negative one from the negative wire towards the centre 

 of the mercury, and a positive one from the positive wire towards 

 the centre of the mercury, the negative one being generally the 

 strongest. If in this 3rd case the mercury contains any impurity, 

 or if a substance be caused by any means to dissolve in the mercury, 

 the movements are notably affected : an electro-positive substance 

 (zinc, alkali-metal, &c.) increases the positive flow so as partly or 

 completely to overpower the negative movement ; and an electro- 

 negative substance increases the negative flow, in a few instances, so 

 as to overpower the positive movement. These influences are also 

 frequently detectable when liquids are used of alkaline or acid reac- 

 tion, as in cases 1 and 2. 



B. When the negative wire is in the mercury and the positive 

 one in the liquid, two cases occur : 1 . With pure mercury, the 

 motion is positive in nearly all liquids, whether acid, alkaline, or 

 neutral ; and 2. With mercury containing a small amount of an 

 electro-negative substance, imparted to it by re versing the connexions 

 of the wires for a short time, a temporary negative flow is produced 

 in certain liquids, chiefly nitrates, but not in certain other liquids. 



C. When the positive wire is in the mercury and the negative 



