A HISTORY OF SCIENCE 



battery of two thousand cells with which he produced 

 a bright light from points of carbon the prototype of 

 the modern arc lamp. He made this demonstration 

 before the members of the Royal Institution in 1810. 

 But the practical utility of such a light for illuminating 

 purposes was still a thing of the future. The expense 

 of constructing and maintaining such an elaborate 

 battery, and the rapid internal destruction of its plates, 

 together with the constant polarization, rendered its 

 use in practical illumination out of the question. It 

 was not until another method of generating electricity 

 was discovered that Davy's demonstration could be 

 turned to practical account. 



In Davy's own account of his experiment he says: 

 "When pieces of charcoal about an inch long and 

 one-sixth of an inch in diameter were brought near each 

 other (within the thirtieth or fortieth of an inch), a 

 bright spark was produced, and more than half the 

 volume of the charcoal became ignited to whiteness; 

 and, by withdrawing the points from each other, a con- 

 stant discharge took place through the heated air, in a 

 space equal to at least four inches, producing a most 

 brilliant ascending arch of light, broad and conical in 

 form in the middle. When any substance was intro- 

 duced into this arch, it instantly became ignited; 

 platina melted as readily in it as wax in a common can- 

 dle; quartz, the sapphire, magnesia, lime, all entered 

 into fusion ; fragments of diamond and points of char- 

 coal and plumbago seemed to evaporate in it, even 

 when the connection was made in the receiver of an 

 air-pump; but there was no evidence of their having 

 previously undergone fusion. When the communica- 



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