Hydrogen from the Anode. 273 



then be clearly distinguished by a pocket-lens. Though I had 

 on this account to abandon analyzing this body, its nature could 

 be inferred from other circumstances. It' the black precipitate, 

 well washed, be placed in dilute acid, it dissolves with evolution 

 of hydrogen. The solution contains no chlorine, so that it was 

 not necessarily the electrolysis of a chloride which had caused 

 the formation of this body ; it is moreover identical with the in- 

 crustation formed in the solution of sulphate of magnesium. A 

 substance which by decomposition of water and development of 

 hydrogen oxidizes to magnesia, could only be magnesium, or a 

 lower oxide of this metal, suboxide of magnesium. As the rela- 

 tion between the amount of the body dissolved and the volume 

 of hydrogen generated could not be determined, this point could 

 not be settled by analytical means. But, apart from the fact 

 that no cause at all can be found why a separation of spongy 

 magnesium should only take place at the positive pole, the pre- 

 cipitate in its electrical comportment resembled an oxide. If 

 the current of a battery be passed through very dilute sulphuric 

 acid between magnesium electrodes, the positive wire becomes 

 immediately black. If the two wires be now connected with a 

 galvanometer by means of the reverser, the positive wire is first 

 positive and then becomes immediately powerfully negative, re- 

 maining so till the black incrustation has vanished. In an 

 experiment the image of the scale of my reflecting galvanometer 

 moved ten divisions to the positive side, but with the next swing 

 of the mirror went over to the fiftieth division on the negative 

 side, remained motionless there, and then went very slowly back 

 to zero, whilst the wire became first covered with white spots 

 and then entirely white. When I first observed this phenome- 

 non I thought I had to do with an action similar to that already 

 noticed in the case of iron*, to which I gave the name of " ano- 

 malous polarization," but which Wiedemannf has called " posi- 

 tive polarization." I soon saw, however, that there Were two 

 different things to be distinguished here — a polarization, and the 

 electromotive action of the black incrustation. The first de- 

 flection, the positive, is to be ascribed to a polarization. This 

 quickly vanishes, and is caused by the development of hydrogen ; 

 for even in dilute acid hydrogen is evolved from the magnesium 

 anode during electrolysis. That the negative plate appears not 

 to be the most strongly positively polarized, but the positive 

 plate covered with a negative layer of oxide, is not without pre- 

 cedent ; for the negative body covered with hydrogen is often 

 more positive than the positive itself: for instance, platinum in 

 a gas-battery, when covered with hydrogen, is more positive than 



* Poggendorff's Annalen, vol. lxiii. p. 415. 

 t Lehre vom Galvanismus, vol. i. p. 50S. 



Phil. Mag. S. 4. Vol. 32. No. 21G. Oct. 1866. T 



