MATTER AND ENERGY. 53 



LIGHT. 



Heat is the manifestation of an energy, and so is 

 light. The glow-worm and the fire-fly flash phos- 

 phorescence on summer nights, arid the " will o' the 

 wisp," or swamp gas, is a torch lighted through the 

 agency of decompositions. The farthing dip feebly 

 illumines the humble cottage, and electric incandes- 

 cence makes brilliant the halls of palaces. When 

 brakes of a swiftly moving train are applied the fric- 

 tion evolves heat enough to kindle a flame, and a 

 spark is elicited by a stroke of flint and steel. Light 

 is emitted in rays from a center of illumination, as the 

 flame of a burning lamp, a blazing meteor, from stars 

 and from the sun. The " fixed stars," so called, issue 

 luminous rays as the sun does, but they are so far 

 away that we see only a stellar twinkle. Light from 

 the moon is wholly reflected, solar rays glancing from 

 the face of the lunar orb to the earth. Sunbeams 

 differ somewhat from other luminous rays. A pencil 

 of sunlight thrown upon a spectrum or glass prism 

 will exhibit plainly the primitive colors, and rays from 

 the electric arc display such hues, but not so promi- 

 nently. Plants do not thrive as well under artificial 

 heat and light as they do under the sun's energies. 

 Solar rays decompose carbonic acid in the leaves of 

 plants through the agency of chlorophyl, the carbon 

 forming woody fiber, while the oxygen disengaged 

 passes into air. All the colored rays of the sun will 

 not decompose the carbonic acid of plants, but the ac- 

 tinic, or chemically active blue and violet rays. Plants 

 may develop in substance without the aid of sun- 

 beams, but the stalks and leaves are colorless, and the 

 acme of maturity can hardly be attained. 



