414 



TENTH ANNUAL REPORT OF 



oxygen liberated at this pole, in consequence of the electroh^sis, does 

 not combine with the iron, but ascends free from it, exactly as though 

 the + pole of the circuit were formed of a platinum wire. Therefore 

 the stratum of oxide which is formed under the above-mentioned cir- 

 cumstances, immediately on the immersion of the iron wire in the 

 liquid, protects it completely from further oxidation. 



Nitric acid of 1.35 sp. gr. is not essential to the success of this ex- 

 periment ; it may be diluted with 100 volumes of water, and yet the 

 iron positive pole immersed in the liquid will become passive, and the 

 oxygen liberated at it will ascend as free gas. 



Precisely similar phenomena take place if dilute sulphuric or phos- 

 phoric acid be used instead of dilute nitric acid. To obtain free oxy- 

 gen at the positive iron wire, the negative pole must be first dipped 

 in the liquid, and then the iron wire connected with the positive pole 

 is placed in it. 



If the positive wire be dipped in the acid before the negative, it 

 will be attacked ; the iron wire will not become passive, if, separated 

 from the positive pole of the battery^ it be dipped in the dihiteacid, 

 no matter whether the negative pole be already in it or not. In short, 

 if iron is to be made passive, the chemical action of the dilute acid 

 raiust not precede the action of the current. 



If, instead of the dilute acid in this experiment, the liquid solution 

 of an oxygen compound be used, which exerts no sensible chemical 

 action on iron, as solutions of alkalis and perfectly neutral salts, the 

 iron will become passive, as though the battery were closed. In using 

 potash lye, or a solution of nitrate of soda, the iron connected with 

 the positive pole will become passive, no ;iiatter whether the negative 

 or positive pole be first placed in the liquid. (Pogg. Ann. XXXVIII, 

 492.) 



Upon this is based the construction of voltameters, which are formed 

 of iron plates immersed in a solution of potash. 



Fig. 40 represents a voltameter 

 ^'s- '^0. constructed of iron platesbyBunsen. 



In a cylindrical glass receiver 6 to 

 8 centimetres in diameter, and 30 

 to 35 centimetres in height, there 

 aretwoconcentric cylinders of sheet- 

 iron, which are kept apart by a sub- 

 stance at once insulating and not 

 liable to be attacked by a solution 

 of potash, such as strands of spun 

 glass. The vessel filled with this 

 solution is closed by a suitable cork 

 through wliich, besides the gas tube, 

 two copper wires pass, each of which 

 is soldered to an iron plate, and put 

 in connexion with the poles of the 

 battery. 



Such a battery having been onoe 

 well constructed, it can be left 

 standing, filled with the potash solution, always ready for use. 



