336 Dynamic Theory. 



miles. When a body is at a temperature of 250 degrees C. it may be 

 called warm ; 500 degrees C. it may be called hot. At 1000 degrees 

 C. we have the heat rays ; at 1200 degrees C. we have the orange rays ; 

 at 1300 degrees C. we have the yellow rays ; at 1500 degrees C. we 

 have the blue rays ; at 1700 degrees C. we have the indigo rays ; at 

 2000 degrees C. we have the violet rays. So that any body raised to a 

 temperature of 2000 C. will give us all the rays of the sun. 



Incandescent electric lighting depends on the principle that a bad 

 conductor becomes heated during the passage of an electric current, and 

 when heated ( to a sufficient degree ) emits light, 1 or rather when the 

 degree of vibration has become great enough the effect of the extra high 

 vibratory rate is not heat, but light. When a carbon rod, a piece of 

 platinum wire, or thin iron wire, forms part of a circuit, it glows with 

 an intensity of light dependent upon the strength of the current and 

 the resistance offered by the bad conductor. If the bad conductor be 

 cut in two and the two halves be brought into contact, the current pass- 

 ing through will give the glow. Now, if the two be slightly separated 

 the current will pass across the separating space producing what is called 

 the voltaic arc. The brilliancy of the arc depends upon the strength of 

 the current, the material composing the electrodes, or points between 

 which the light is produced, and the atmosphere about it. With potas- 

 sium or sodium electrodes the light is more brilliant than with plat- 

 inum or gold. With sodium the color of the arc is yellow, with zinc 

 and magnesium it is white, while with silver it is green. The 2 spec- 

 trum of the arc produced between silver and carbon contains only two 

 green bands, and if other metals be substituted for the silver, the spec- 

 trum is always ''discontinuous " being a few color bands separated by 

 dark spaces. ( The spectra of gas and oil flames are continuous, but 

 red, orange and yellow are predominant, there being little green, less 

 blue and still less violet, or none at all. ) In the electric arc, where 

 both points are carbons, the light from the carbons is white, the same 

 as sunlight, that is, contains all the colors, while the light of the arc 

 itself is violet^blue, its spectrum containing an excess of violet, less of 

 blue, and scarcety any of the colors of the lower end of the spectrum. 

 The electric light depends for its direct development upon mechanical 

 energy and not upon combustion, as do gas, oil, &c. , consequently no 

 carbonic acid gas results from it. The chemical action of the electric 

 light is the same as that of the sun. It causes the combination of 

 chlorine with hydrogen, decomposes chloride of silver, and can conse- 

 quently be utilized in photography, and it imparts phosphorescent 

 properties to susceptible substances. It takes two and a half times 



1 See Paget Higgs Electric Light, p. 55. 



2 For spectrum and spectrum analysis see Chap, on Light. 



