46 



ELECTRICITY. 



strong shock passed through it, the 

 metal will disappear with a bright flash, 

 and the impulse with which its particles 

 are driven against the paper will produce 

 a permanent stain of a purple or grey 

 colour. Franklin found that if the 

 metallic leaf be placed between two 

 panes of glass firmly tied together, the 

 explosion, provided the glass withstands 

 the concussion, will leave on each of 

 its surfaces an indelible stain, in conse- 

 quence of some of the metallic particles 

 being actually forced into the substance 

 of the glass, and being then inaccessible 

 to the action of chemical solvents applied 

 to the surface of the glass. Sometimes 

 it is found that these metallic stains 

 extend to a greater distance than the 

 breadth of the piece of metal. It often 

 happens, however, that the pieces of 

 glass themselves are shattered to pieces 

 by the discharge. 



(169.) The colours produced by the 

 electric explosion of metals have been 

 applied to impress letters or ornamental 

 devices on silk and on paper. For this 

 purpose Mr. Singer directs that the out- 

 line of the required figure should be 

 first traced on thick drawing paper, and 

 afterwards cut out in the manner of 

 stencil plates. The drawing paper is 

 then placed on the silk or paper intended 

 to be marked ; a leaf of gold is laid upon 

 it, and a card over that ; the whole is 

 then placed in a press or under a weight, 

 and a charge from a battery sent through 

 the gold leaf. The stain is confined by 

 the interposition of the drawing paper to 

 the limit of the design, and in this way a 

 profile, a flower, or any other outline 

 figure may be very neatly impressed. 



(170.) The heat evolved by electricity, 

 like most other of its effects, is in pro- 

 portion to the resistances opposed to its 

 passage. The less the conducting power 

 of a metal, the greater is the portion of 

 it which the same shock can ignite or 

 destroy. A rod of wood of considerable 

 thickness being made part of the circuit, 

 has its temperature sensibly raised by a 

 very few discharges. Most combustible 

 bodies are capable of being inflamed by 

 electricity, but more especially if it be 

 made to strike against them in the form 

 of a spark or shock obtained by an in- 

 terrupted circuit, as by the interposition 

 of a stratum of air. In this way may 

 alcohol, ether, camphor, powdered resin, 

 phosphorus, or gunpowder be set fire 

 to. The inflammation of oil of turpen- 

 tine will be promoted by strewing upon 

 it fine particles of brass filings, If the 



spirit of wine be not highly rectified, it 

 will generally be necessary previously to 

 warm it, and the same precaution must 

 be taken with other fluids, as oil and 

 pitch ; but it is not required with ether, 

 which usually inflames very readily. 

 But, on the other hand, it is to be re- 

 marked that the temperature of the body 

 which communicates the spark appears 

 to have no sensible influence on the heat 

 produced by it. Thus the sparks taken 

 from a piece of ice are as capable of in- 

 flaming bodies as those from a piece of 

 red-hot iron. Nor is the heating power 

 of electricity in the smallest degree dimi- 

 nished by its being conducted through 

 any number of freezing mixtures which 

 are rapidly absorbing heat from sur- 

 rounding bodies. 



(171.) Light, as well as heat, is emit- 

 ted during the electric discharge at every 

 point where the circuit is either inter- 

 rupted, or is occupied by bodies of infe- 

 rior conducting powers. A moderate 

 charge will produce a bright spark when 

 made to pass through water, and the 

 spark is still more luminous in oil, alco- 

 hol, or ether, which are worse conductors 

 than water : on the contrary, in fluids of 

 greater conducting power there is greater 

 difficulty of eliciting electric light. Thus 

 a much higher charge is required to 

 produce a spark in hot water than in 

 cold ; a still higher in saline solutions ; 

 and in concentrated acids, light can be 

 obtained only when their volume is very 

 small ; so that it is necessary for that 

 purpose, to draw a line of the acid upon 

 a plate of glass with a camel's hair 

 pencil. This is illustrated by the follow- 

 ing experiment mentioned by Singer. 

 Draw a line with a pen dipped in water 

 on the surface of a slip of glass ; place 

 one extremity of the line in contact with 

 the coating of a Leyden jar, and at six 

 inches distance upon the line place one 

 knob of the discharging rod ; when the 

 jar is fully charged, bring the other ball 

 of the discharger to the knob of the jar, 

 and the discharge will take place lumi- 

 nously over the six inches of water. Next, 

 trace a line with a pen dipped in sul- 

 phuric acid on a slip of glass, as in the 

 former experiment, and place one extre- 

 mity of it in contact with the outside of 

 the jar; the ball of the discharger may 

 then be placed on the glass at twelve 

 inches distance, and the electric fluid 

 will pass as brilliantly over that interval 

 as over the six inches of water. In 

 either of these experiments, if the line of 

 fluid be wider in any particular part, the 



