294 WELLS'S NATURAL PHILOSOPHY. 



What are the ^he cLief souices of light are the sun, the 

 of iightr"'^''^* stars, fire or chemical action, electricity, and 

 phosphorescence. 



Under the head of chemical action are included all the forms of artificial 

 light which are obtained by the burning of bodies. Examples of light pro- 

 duced by pliosphorescence, as it is called, are seen in the glow of old and d> 

 cayed wood, and in the light emitted by fire-flies and some marine animals. 



G42. All bodies are either luminous or non-luminous. 

 What is a lu- Luminous bodies are those which shine by 

 mmousbody? ^jjgjj, ^^^.^ light; such, for example, as the 

 BUn, the flame of a candle, metal rendered red hot, etc. 



All sohd bodies, when exposed to a sufficient degree of heat, become lu- 

 minous. It has been recently proved* that all solids begin to emit light at 

 the same degree of heat, viz., 977° of Fahrenheit's thermometer. As the 

 temperature rises, the brilliancy of the light rapidly increases, so that at a 

 temperature of 2000° it is almost forty times as intense as at 1900°. Gases 

 must be heated to a much greater extent before they begin to emit light. 



What is a. non- Nou-lumlnous bodlcs are those which pro- 

 luminousbody? ^^^^^ ^^ jjg|j^ thcmselves, but which may be 



rendered temporarily luminous by being placed in the 

 presence of luminous bodies. 



Thus, the sun, or a candle, renders objects in an apartment luminous, and 

 therefore visible ; but the moment the sun or candle is withdrawn, they be- 

 come invisible. 



What are trans- Transparent bodies are those which do not 

 parent bodies? interrupt the passage of light, or which allow 

 other bodies to be seen through them. Glass, air, and 

 water are examples of very transparent bodies. 

 What are Opaquc bodics are those which do not permit 

 opaque bodies? ^ight to pass through them. The metals, 

 stone, earth, wood, etc., are examples of opaque bodies. 



Transparency and opacity exist in different bodies in very different degrees. 

 We can not clearly explain what there is in the constitution of one mass of 

 matter, as compared with another, which fits the one to transmit light, and 

 the other to obstruct it ; but the arrangement of the particles has undoubt- 

 edly much influence. 



Strictly speaking, there is no body which is perfectly transparent, or per- 

 fectly opaque. Some light is evidently lost in passing even through space, 

 and still more in tr&versing our atmosphere. It has been calculated that the 

 atmosphere, when the rays of the sun pass perpendicularly through it, inter- 

 • By Prof. J. W. Draper. 



