CHEMICAL INFLUENCE OP LIGHT. 281 



chloride of silver. In Fig. 104 (p. 280) you have the " posi- 

 tive " of the same leaf. 



396. Fixing the Picture. The figures of which we have 

 spoken can not be permanent, for exposure to light will 

 destroy them by making the whole surface equally dark. 

 To verify this, take a copy of lace and hold it up to a win- 

 dow. The white lines of the tracery will disappear quick- 

 ly, the whole surface being subjected to the chemical power 

 of the light, and becoming therefore covered with the dark 

 silver. This effect, however, will not occur if the window 

 be covered with a heavy yellow curtain, for this will not 

 allow the chemical power to pass through. Now if after 

 a picture is made we could by means of any substance re- 

 move from it all the chloride of silver, and at the same time 

 leave the metallic silver untouched, we should have a pict- 

 ure which the light can not affect. Such a substance we 

 have in sodium hyposulphite. This dissolves out the un- 

 decomposed chloride of silver, but produces little or no 

 effect upon the metallic silver which constitutes the parts 

 of a " positive " picture. 



397. Photography. We have given you a brief outline of 

 the principles on which the beautiful art of photography 

 is based. To pursue this interesting subject any farther 

 will lead us too deeply into this important branch of Ap- 

 plied Chemistry. 



QUESTIONS. 



388. Give some examples of the chemical influence of light. 389. What 

 is said of the universality of its influence ? 390. State in full what is said 

 of its influence on vegetables. 391. Give the anecdote of George Stephen- 

 son. 392. State in full what is said of the influence of light on animals. 

 393. What three powers are there in the sun's rays ? Show how these are 

 arranged in the spectrum. Why can they be thus partially separated ? 

 What is actinism ? Where in the spectrum is the point of greatest light ? 



