CHEMICAL INFLUENCE OF LIGHT. 277 



393. Light Dissected. Light that does all this is not 

 one thing ; but in every ray, besides the seven colors which, 

 blended together, make the white light, there are two dis- 

 tinct powers heat and chemical power. The dissection 

 of light effected by the prism is depicted in Fig. 102. We 



Proportionate width of th ^^^ .... Greatest Chemical Action. 



of Colors* 



Yellow 40 | I ....Greatest Light. 



Greatest Heat 

 Fig. 102. 



have in Part I. described the manner in which this spec- 

 trum, so called, is made, and commented upon the colors 

 that compose white light, and we need to say no more hero 

 on these points. The chemical power is what concerns us 

 now. This is greatest at the violet end of the spectrum, 

 diminishing as you go from there toward the other end. 

 The greatest heat, on the other hand, is at the red end. 

 We have, then, bound up in every ray of light that comes 

 from the sun three powers viz., light, heat, and chemical 

 power. This last has been called actinism, or the actinic 

 power. The reason that these three powers can be par- 

 tially separated in the spectrum, as well as the different 

 colors, is that different parts of the ray are differently re- 

 frangible the calorific part the least, the actinic part the 

 most, and the illuminating part between the two. Then 

 of the colors, the least refrangible is the red, the most so 

 the violet. You observe that we speak of the separation of 



