122 ELEMENTS OF LABORATORY WORK 



that no gas escapes. The evolution of gas will cease after a 

 time, but will continue again if the cork is removed. The 

 presence of a relatively large quantity of the gas in the con- 

 fined space evidently prevents further action. It is advisable 

 not to cork the tube when the action is very rapid, and to watch 

 the tube, when corked, from some distance. 



80. Decomposition takes place when Silver Nitrate, or 

 Similar Bodies, either Liquefied or in Solution, forms part of 

 an Electric Circuit. Some silver nitrate is dissolved in pure 

 water, and, when placed in a suitable vessel, made to form part 

 of an electric circuit. Connection with the circuit is. made by 



B.ND.NG SCREWS ***ff*g the Solution 



WOOD * wo c PP er pl a tes, of which 

 the weights have been 

 taken, and attaching them, 

 as shown (fig. 42), to the 

 wires leading to the cells, 

 by means of hooks and 

 binding-screws fixed in a 

 wooden support. Plates 

 are used in order that the 

 portion of the solution 

 added to the circuit may 

 have a large sectional area. 



It has been shown that conductors of small sectional area 

 diminish the stress of the circuit. 



After a few minutes it will be found that one of the copper 

 plates has become coated with a white metal, which is evidently 

 silver. On carefully drying and weighing this plate it will be 

 found heavier. 



Some silver nitrate is raised in temperature so as to liquefy 

 without being decomposed, and then connected with the circuit 

 by two wires dipping into it. After a short time a bead of 

 silver will be found at the end of one of the wires. If solid 

 silver nitrate is used no decomposition takes place. The 

 liquid condition is essential to electric decomposition or electro- 

 lysis. If a solution of copper sulphate is substituted for that 

 of silver nitrate in the first experiment, a similar change will be 



