ITS EEL ATI OX TO HEAT. 



inquiry whether the chemical action produced by light upon 

 certain bodies was merely the effect of the heat accompanying it, 

 or owing to some other cause. By a series of delicate experi- 

 ments, Berard found that this action is not only independent of 

 the heating power, but follows entirely a different law ; its 

 intensity being greater in the violet ray, where the heating power 

 is the least, and least in the red ray, where the heating power is 

 the greatest. TTe are thus led to the conclusion that the solar 

 rays possess at least three distinct powers those of heating, 

 illuminating, and effecting chemical combinations and decom- 

 positions ; and these powers are distributed among the different 

 refrangible rays in such a manner as to show their complete 

 independence of each other. 



11. In relation to the production of light, bodies are considered 

 as luminous and non-luminous. 



Luminous bodies, or luminaries, are those which are original 

 sources of light, such, for example, as the sun, the flame of a 

 lamp or candle, metal rendered red-hot, the electric spark, 

 lightning, and so forth. 



Luminaries are necessarily always visible when present, pro- 

 vided the light they emit be strong enough to excite the eye. 



Non-luminous bodies are those which themselves produce no 

 light, but which may be rendered temporarily luminous when 

 placed in the presence of luminous bodies. These cease, how- 

 ever, to be luminous, and therefore visible, the moment the 

 luminary from which they borrow their light is removed. Thus 

 the sun, placed in the midst of the planets, satellites, and comets, 

 renders these bodies luminous and visible ; but when any of 

 them is removed from the solar influence by the interposition of 

 any object not pervious by light, they cease to be visible, as is 

 manifest in the case of lunar eclipses, when the globe of the 

 earth is interposed between the sun and moon, and the latter 

 object is therefore deprived of light. A candle or lamp placed 

 in the room renders the walls, furniture, and surrounding ob- 

 jects temporarily luminous, and therefore visible ; but if the 

 candle be screened by any object not pervious to light, those 

 parts of the room from which light is intercepted would become 

 invisible, did they not receive some light from the other parts 

 of the room still illuminated. If, however, the candle or lamp be 

 completely covered, all the objects in the room become invisible. 



12. In relation to the propagation of light, bodies are con- 

 sidered as transparent and opaque. Bodies through which light 

 passes freely are called transparent, because the eye placed 

 behind them will see such light through them. Bodies, on the 

 contrary, which do not admit light to pass through them, are 



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