LUMINOUS EFFECTS OF THE DISCHARGE. 635 



end to one arm of a common discharger. As soon as the battery is 

 strongly charged, the knob of the other arm is quickly brought 

 close to a knob of one of the jars, and if the experiment is success- 

 ful the whole strip of tinfoil will be fused and burn with a flash of 

 light, being converted into oxide of tin, which is seen as a white 

 cloud between the ends of the wire. If the experiment is only 

 partially successful the tinfoil melts only in one or two places. 



Even fine iron wire may be burnt by the discharge of a battery. 

 Of the common iron wire sold, even the thinnest sort, which has a 

 diameter of about O mm> 2, is too thick for the experiment ; but it can 

 be made thinner by the action of nitric acid, which dissolves 

 the iron gradually. A piece of wire, 10 cm long, is bent at both ends 

 into small loops, after softening over a spirit-lamp (not over the 

 Bunsen flame, which burns the wire) those portions of the wire only 

 which are required for the loops ; if the middle portion is heated, 

 the nitric acid will afterwards not properly act upon it. The middle 

 portion is placed in a mixture of 3 CC of nitric acid and 30 CC of 

 water, contained in a small saucer, and left there until it is as thin 

 as possible. The wire is then well washed by a jet from the wash- 

 ing-bottle, and suspended by the loops to the rings of the two wires 

 of the universal discharger, the wires being for this experiment 

 reversed in the corks so as to turn the ends with the rings towards 

 each other. The discharge is conducted precisely as in the experi- 

 ment with the tinfoil strip. The iron wire also is sometimes only 

 burnt in separate places ; but when the experiment succeeds com- 

 pletely the whole is burnt and converted into small fused drops of 

 iron oxide, which fly about in numerous scintillating sparks. 



Care is required in handling the acid ; if a drop should get on to 

 the clothes it should be immediately touched with solution of 

 ammonia carbonate, which it is best to have in readiness, for as 

 soon as the spot becomes visible it is not easy to remove it. 

 The hands are coloured yellow by nitric acid, and the colour dis- 

 appears only gradually as the destroyed skin peels off and is replaced 

 by a new growth underneath it. 



The spark which accompanies the recombination of 

 the two electricities is nothing else than the light 

 emitted by the particles of air through which the dis- 

 charge takes place. Air is a non-conductor, and is 

 heated so as to become luminous even by a faint dis- 

 charge. If the discharge is allowed to take place in 



