2 Professor Tyndall [Jan. 17^ 



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.* 



The first condition to be fulfilled in the development 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 

 unwarmed an atmosj)heric discharge competent to shiver to splinters 

 a resisting oak. I send the self-same current through a wire composed 

 of alternate lengths of silver and platinum. The silver offers little 

 resistance, the platinum offers much. The consequence is that the 

 platinum is raised to a white heat, while the silver is not visibly warmed. 

 The same holds good with regard to the carbon terminals employed for 

 the production of the electric light. The interval between the 

 terminals offers a i^owerful resistance to the passage of the current, 

 and it is by the gathering up of the force necessary to burst across 

 this interval that the voltaic current is able to throw the carbon into 

 that state of violent intestine commotion which we call heat, and to 

 which its effulgence is due. 



The smallest interval of air usually suffices to stop the current. But 

 when the carbon points are first brought together and then separated, 

 there occurs between them a discharge of incandescent matter which 

 carries, or may carry, the current over a considerable space. The 

 vapours of the metals, for example, yield arcs of extraordinary length. 

 When a pellet of silver is substituted for the positive carbon, an arc of 

 incandescent silver vapour is obtained many times the length of that 

 obtainable between the pure carbons. The part played by resistance 

 is strikingly illustrated by the deportment of silver and thallium 

 when mixed together and volatilized in the arc. The current first 

 selects as its carrier the most volatile metal, which in this case is 

 tb allium. While it continues abundant, the passage of the current is 

 so free — the resistance to it is so small — that the heat generated is 

 incompetent to volatilize the silver, f As the thallium disappears the 

 current is forced to concentrate its power; it presses the silver into its 

 service, and finally fills the space between the carbons with a vapour 

 which, as long as the necessary resistance is absent, it is incomj^etont 

 to produce. 



* In the concluding lecture at the Royal Institution in June, 1810, Davy fused 

 iridium, tlie alloy of iridium and osmium, and other refractory suhstances. See 

 ' Philosophical Magazine,' vol. H5, p. 4G3. Quetelet assigns the first production 

 of tiie 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. IGG. 



t I have already drawn attention to a danger which besets the spectroscopist 

 when operating upon a mixture of constituents volatile in different degrees. 

 When, ia 1872, I lirst observed the effect (lescril)ed in the text, had I not known 

 that fcilver was present, I should have inferred its absence. 



