SOURCES OF LIGHT. 289 



the light emitted whilst hot. Electricity developed in nature also 

 produces visible light under suitable conditions, e.g., the lightning 

 flash and the aurora borealis ; whilst chemical action also occurs 

 naturally under such conditions as to develop visible light ; in some 

 instances, in consequence of great heat being produced (e.g., where 

 natural petroleum or other combustible matter is fired) ; in others 

 without the production of any notable amount of heat, e.g., in the 

 case of the glow-worm and " phosphorescent " animal and vegetable 

 decaying matter, such as fish, certain kinds of wood, marshy 

 emanations (Will of the wisp), &c., &c. 



Of the artificial sources of light the two chief ories employed 

 are, firstly, those where light is produced by means of electricity, 

 itself generated either by chemical action (directly, as in the voltaic 

 battery ; indirectly, as when fuel is burnt and motive power then 

 developed by means of a gas engine, steam engine, or other analo- 

 gous appliance), or by utilising natural sources of mechanical 

 power, such as the flow of water from a higher to a lower level, the 

 tides, the pressure of the wind, &c.; the mechanical power being 

 transformed into electricity by one or other of a variety of forms 

 of a machine now generally known as a "dynamo." Secondly, 

 those produced by the agency of heat developed by chemical action, 

 ordinary candle flames, oil and gas lamps, furnaces and fires, the 

 lime light (Expt. 211), and fireworks, &c., being all cases of this 

 kind. Sparks produced by percussion (such as the flint and 

 steel mill, where a feeble light is developed by the continual 

 striking together of pieces of steel and fragments of flint, &c.), 

 and phosphorescent action produced by friction, &c. (as when 

 quartz pebbles are rubbed together or loaf sugar broken up in 

 the dark, or on 'heating fragments of the mineral termed fluor 

 spar on account of its use as a flux in metallurgical operations 

 vide also Expt. 271), are also processes by which visible light is 

 developed without the occurrence of chemical changes. Certain 

 bodies possess the property of absorbing and storing up light, so 

 to speak, so that when exposed for a short time to a tolerably 

 bright light, and then removed to the dark, they glow and become 

 luminous, emitting again the light absorbed during the previous 

 exposure. Many articles are now sold prepared with a " luminous 

 paint" of this kind, consisting of a solid phosphorescent body 

 (usually a preparation containing sulphide of calcium) ground to 

 powder and mixed with oil, so that the articles will glow at night 

 in virtue of having been exposed to light during the day. 



Certain kinds of radiant energy which are capable of exerting 

 energetic chemical action (Chapter XXY.) are not perceptible to 

 the ordinary human eye ; but various substances have the power, 



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