420 



FRAGMENTS OF SCIENCE. 



the voltaic circuit attracted marked attention, and in 

 the innumerable tests and trials to which this question 

 was subjected, the utility of platinum and charcoal as 

 means of exalting the light was on all hands recognised. 

 Mr. Children, with a battery surpassing in strength all 

 its predecessors, fused platinum wires eighteen inches 

 long, while ' points of charcoal produced a light so 

 vivid that the sunshine, compared with it, appeared 

 feeble.' l Such effects reached their culmination when, 

 in 1808, through the liberality of a few members of 

 the Royal Institution, Davy was enabled to construct a 

 battery of two thousand pairs of plates, with which he 

 afterwards obtained calorific and luminous effects far 

 transcending anything previously observed. The arc 

 of flame between the carbon terminals was four inches 

 long, and by its heat quartz, sapphire, magnesia, and 

 lime, were melted like wax in a candle flame ; while 

 fragments of diamond and plumbago rapidly disappeared 

 as if reduced to vapour. 8 



The first condition to be fulfilled in the develop- 

 ment of heat and light by the electric current is that 

 it shall encounter and overcome resistance. Flowing 

 through a perfect conductor, no matter what the 

 strength of the current might be, neither heat nor light 

 could be developed. A rod of unresisting copper 

 carries away uninjured and un warmed an atmospheric 

 discharge competent to shiver to splinters a resisting 



1 Davy, ' Chemical Philosophy,' p. 110. 



* In the concluding lecture at the Eoyal Institution in June, 

 1810, Davy showed the action of this battery. He then fused 

 iridium, the alloy of iridium and osmium, and other refractory sub- 

 stances. * Philosophical Magazine,' vol. xxxv. p. 463. Quetelet 

 assigns the first production of the spark between coal-points to 

 Curtet in 1802. Davy certainly in that year showed the carbon 

 light with a battery of 150 pairs of plates in the theatre of the 

 Royal Institution ( * Jour. Roy. Inst. ' vol. i. p. 166), 



