1867.] Physics. 285 



use, are always pointed either due north or a few degrees towards 

 the west ; in fact, nearly in the direction indicated by a compass 

 needle, — so that the repeated shocks brought about by the explosion 

 of the powder may be considered equivalent to so many hard blows 

 from a hammer, which are known to have a similar effect. The 

 magnetic character appears to be permanent^ which would not be 

 the case if the gunbarrels were of the softest description of mal- 

 leable iron ; and the region of the breech is, in every instance, 

 possessed of north polarity, since it strongly attracts the south pole 

 of the compass needle. These effects would probably not be noticed 

 at all, or only to an inferior degree, in arms ordinarily fired, in 

 directions east and west ; and it would be imagined that by revers- 

 ing the usual practice, if it were possible, and firing towards the 

 south, the indications of polarity would be changed. 



The foregoing observation seems to have a parallel in the 

 remarkable instance of the polarity of Her Majesty's ironclad 

 ' Northumberland,' built last year at Millwall. This vessel was 

 placed due north and south during her construction, and the 

 repeated vibrations to which she was subject whilst in that position 

 induced a condition of polarity which demanded a subsequent opera- 

 tion for the purpose of demagnetizing her. No such practical 

 inconvenience arose in the case of the iron steamship 'Great 

 Eastern,' which was built in the same yard, but in a position at 

 right angles to the former, or nearly due east and west. 



M. Eoullion proposes a new battery based upon some observa- 

 tions he has made on the action of aqua regia on silver. A mixture 

 consisting of two-thirds hydrochloric and one-third nitric, or three- 

 fifths hydrochloric and two-fifths nitric acids will easily dissolve 

 gold and platinum, but will only superficially attack pure unalloyed 

 silver ; a superficial chloride being formed, which protects the rest 

 of the silver like an impermeable varnish, however long it may 

 remain in the aqua regia. If copper be present, the metal is 

 attacked. M. Eoullion has utilized this fact to make a new battery, 

 in which pure silver in aqua regia replaces the platinum or carbon 

 in the nitric acid of a Grove's or Bunsen's cell. He says that after 

 several months' use, the silver has not sensibly diminished in 

 volume, and no chloride of silver has been found in the porous 

 cell. He considers this battery to be more constant than Bunsen's 

 battery. 



The gas battery of Mr. Grove has received some attention from 

 M. J. M. Gougoin. He arrives at conclusions in perfect confor- 

 mity with those of M. Schonbein. Mr. Grove supposes that it is 

 indispensable for each of the electrodes of platinum of his gas- 

 couple to be simultaneously in contact with one of the gases and 

 with the liquid placed underneath. M Gougoin states, on the con- 



