COLOURS OF LIGHT. 127 



times in the morning, and often gives the last tint 

 to the clear sky. 



Now there is most heat in the red end ; that 

 heat is greater without, or on the edge of the 

 colour, than where it is most intense ; and it 

 diminishes as the blue end is approached, so as to 

 be barely, if at all, perceptible there. He is the 

 grand agent in burning, the result of which is the 

 union of the whole or part of the substance burned 

 with that part of the atmospheric air which is 

 called oxygen; and it also favours the union of 

 oxygen with substances when there is merely heat 

 but no flame. Substances which are combined 

 with oxygen are said to be oxydized ; the red end 

 of the spectrum, which heats the most, also oxy- 

 dizes the most ; and that property becomes less 

 and less till the middle is arrived at, and there it 

 is not perceptible even by the nicest tests. That 

 middle is in the green, just about that shade of it 

 which we call grass green, and seen in a well-kept 

 lawn of fine forest grasses. At the blue or most 

 distant end, there is a property the very opposite 

 of that at the red; and, like the former, it is 

 strongest without, or at the edge of the colour, 

 and it becomes less and less till where the green 

 is reached, it is as imperceptible as that which begins 

 at the red end. The property which begins at the 

 blue end hinders oxydation, and in some cases 

 restores oxydized bodies to their former state. 



When vegetables are in a state of vigorous and 

 healthy action, they absorb or drink up the red 

 rays of the sun's light, and return the green to the 

 eye ; and the red light, or that invisible oxydiz- 

 ing agency which accompanies the red end of 

 the spectrum, facilitates the combination of the 

 oxygen of the atmosphere with the surplus carbon 



