■ THE ANALYSIS OF LIGHT. 331 



May the color 691. Bj changing the structure or molecu- 

 cLil'ged^^ by lar arrangement of a body, the color which it 

 moiecuiar'^^*' cxhibits may be often changed also. 



® Illustrations of this principle are frequently Been in chem- 



ical compounda The iodide of mercury is a beautiful scarlet compound, which, 

 when gently heated, becomes a bright yellow, and so remains when undis- 

 turbed. If| however, it is touched, or scratched with a hard substance, ag 

 with the point of a pin, its particles turn over, or readjust themselves, and 

 resume their original red color. Chameleon mineral is a solid substance pro- 

 duced by fusing manganese with potash ; when dissolved in water, it changes, 

 jccording to the amount of dilution, from green to blue and purple. Indigo 

 also, spread on paper and exposed to heat, becomes red. 



692. Some bodies have the power of reflecting from their 

 surfaces one color while they transmit another. 



This is the case with the precious opal. A solution of quinine in water 

 containing a little sulphuric acid, is colorless and transparent to the eye look- 

 ing through it, but by looking at it, it appears intensely blue. An oil ob- 

 tained in the distillation of resin transmits yellow light, but reflects violet 

 light. Smoke reflects blue hght, but transmits red light. These phenomena 

 result from a peculiar action of the surface or outer layer of the substance 

 of the body on some of the rays of light entering it, and have received the 

 name of epipolic, or surface dispersion. 



Deepness of color proceeds from a deficiency, rather than from an abund- 

 ance of reflected rays : thus, if a body reflects only ft few of the red rays, it 

 will appear of a dark red color. When a great number of rays are reflected, 

 the color will appear bright and intense. 



If the objects of the material world had been illuminated only with whito 

 light, all the particles of which possessed the same degree of refrangibility, 

 and were equally acted upon by all substances, the general appearance of 

 nature would have been dull, and all the combinations of external objects, 

 and all the features of the human countenance would have exhibited no other 

 variety than that which they posses in a pencil sketch or India-ink drawing. 



What are com- 693. Any two colors which are able, by com- 

 piementary biuiiig, to produco whitc light, arc termed 

 complementary colors. 

 Each color of the solar ray has its complementary color, 

 for if it be not white, it is deficient in certain rays that 

 would aid in producing white. And these absent rays 

 compose its complementary color. 



The relative position of complementary colors in the prismatic spectrum may 

 be determined as follows* Thus, if we take half the length of a spectrum by 

 fr pair of compasses, and fix one leg on any color, the other leg will fall upoa 



