278 CHEMISTRY. 



the three forces or powers in the spectrum as being par- 

 tial. We will explain this. As the ray of light is bent out 

 of its course by the prism, and the spectrum is formed on 

 the screen, each of the three powers has a point in the spec- 

 trum where its influence is most concentrated. On each 

 side of this it lessens till you come to a point where it is 

 comparatively feeble. "The result of the action of any 

 ray depends, however, greatly on the physical state of the 

 surface upon which it falls and in the chemical constitution 

 of the body ; indeed, for every kind of ray a substance may 

 be found which under particular circumstances will be af- 

 fected by it; and thus it appears that the chemical func- 

 tions are by no means confined to any set of rays to the 

 exclusion of the rest " (Fownes). 



394. Experiments. Many interesting experiments can be 

 tried with the spectrum, some of which we will detail : 



Brush over some paper with a solution of nitrate of sil- 

 ver, and then expose strips of it to different parts of the 

 spectrum. A strip applied at the lower part where the red 

 color or ray is will be scarcely affected, for the chemical 

 rays there have little or no power to affect this substance. 

 A strip at the violet end, on the other hand, will be dark- 

 ened quite rapidly, because there is the centre of the influ- 

 ence of this power. So, also, a strip in the green ray will 

 not be affected so much as one in the blue, because the lat- 

 ter is nearer to that centre. 



Paper charged with chloride of silver is still more sensi- 

 tive to light than that charged with the nitrate, and there- 

 fore gives more decisive results. It may be charged in the 

 following manner: The paper is first wet in a solution of 

 common salt or chloride of sodium. Then it is brushed 

 over with a solution of nitrate of silver. This decomposes 

 the salt, leaving on the paper chloride of silver in place of 

 the chloride of sodium : 



