12 



GALVANISM, 



tensity, under water, alcohol, ether, oils, 

 and other fluids of inferior conducting 

 power. 

 3. Evolution of Heat by Galvanism. 



(29.) The evolution of heat is one of 

 the effects which accompany the action 

 of the voltaic, as well as of the electric 

 battery ; but there is a remarkable dif- 

 ference in the circumstances which favour 

 its production in the two cases. In the 

 common electrical apparatus, heat is not 

 sensibly evolved where the electricity 

 moves with perfect freedom, but only 

 when some resistance is opposed to its 

 passage, and when there is a sudden 

 restoration of its equilibrium, accom- 

 panied with light and sound. But in 

 the voltaic battery, an elevation of tem- 

 perature is observed to take place when 

 the circuit remains complete, when no 

 light is evolved, and when the stream 

 of electricity is conducted in the most 

 silent manner. That the mere passage 

 of voltaic electricity through bodies 

 raises their temperature, is proved by 

 making a wire, forming part of the cir- 

 cuit, pass through a known quantity of 

 water, contained in a vessel, with a 

 thermometer immersed in the fluid. The 

 heat acquired by the water soon be- 

 comes sensible by the rise in the ther- 

 mometer, which even attains the boiling 

 point ; and the water continues in ebul- 

 lition as long as the experiment is con- 

 tinued. 



(30.) The circulation of voltaic elec- 

 tricity produces an elevation of tempe- 

 rature, not only in that part of the 

 circuit which connects together the poles 

 of the battery, but also in the battery 

 itself, every part of which, both the 

 plates of metal and the fluid in the cells, 

 become heated when the apparatus is 

 in an active state. But the elevation of 

 temperature is found not to be equal 

 throughout the series ; and the ditfer- 

 ence is dependant on causes which have 

 not yet been accurately determined. 

 Mr. John Murray found a gradual in- 

 crease of temperature in the successive 

 cells from the negative to the positive 

 pole ; and when a number of different 

 troughs were joined together, the cells 

 at the extremities of each were less 

 heated than those towards the middle ; 

 the maximum of heat was at a part 

 situated nearer to the positive pole ; and 

 the temperature gradually diminished 

 in the direction of the negative pole.* 

 (31.) Ignition, in various degrees, is 



* Edinburgh Philosophical Journal, xiv. 57. 



produced by the" passage of voltaic elec- 

 tricity through metallic wires, when their 

 size and length are properly propor- 

 tioned to the kind of apparatus, and to 

 the quantity of electric fluid they have 

 to convey. Iron wire is in general easy 

 to ignite, and is often fused into glo- 

 bules ; and steel wire is made to burn 

 with a rapid and brilliant combustion. 

 A wire of platina, a metal not suscepti- 

 ble of being acted on by the air, may be 

 kept at a red, or even white heat, for an 

 indefinite length of time, by voltaic elec- 

 tricity. As long, indeed, asj the battery 

 retains its power, there appears to be no 

 limit to the continual evolution of heat. 



(32.) The order in which the different 

 metals are raised to a red heat by the 

 action of galvanism, was ascertained by 

 Mr. Children, with the aid of a very 

 powerful apparatus of his own con- 

 struction, to be as follows, namely, pla- 

 tina, iron, copper, gold, zinc, silver. 

 Between copper and gold the difference 

 is inconsiderable; and with regard to 

 platina and iron, their relative places in 

 the scale seem to depend upon the tem- 

 perature acquired. The relations of tin 

 and lead to the other metals could not 

 be ascertained in these experiments, on 

 account of their melting before they 

 could be raised to a red heat. A beau- 

 tiful illustration of the difference exist- 

 ing in metals as to their capacity of 

 ignition, is obtained by placing in the 

 circuit a wire or chain composed of 

 alternate portions, or links of platina 

 and silver soldered together ; it will then 

 be found that the silver links are not 

 sensibly heated, while all those of pla- 

 tina become equally and intensely ig- 

 nited. 



(33.) It would appear that the heat 

 produced by the voltaic battery is more 

 intense than can be excited by any other 

 process. In the experiments detailed 

 by Mr. Children,* the action of his 

 powerful apparatus raised to a red heat, 

 visible in full daylight, the whole of a 

 wire of platina, one tenth of an inch in 

 diameter, and five feet and a half in 

 length. It also effected the fusion of a 

 variety of substances on which the heat 

 of the best wind- furnaces makes no im- 

 pression. 



(34.) When very thin metallic leaves 

 are placed in the electric current of a 

 powerful voltaic battery they take fire, 

 and by continuing the action, may be 

 made to burn with great brilliancy. In 



* In thtj Philosophical Transactions for 1815, p, 

 368-370, 



