82 COLOR CHANGES THE QUALITIES OF BODIES. 



the steel, that if it be taken from the flame and immediately 

 quenched in tallow, whilst it- is yellow, it will he of such a hard- 

 ness, as renders it appropriate for drills ; hut if kept for a few 

 minutes longer in the flame till it turns blue, it becomes much 

 softer, and is suitable to springs for watches, which are there- 

 fore commonly of that color. Lastly, if kept in the flame until 

 the deep blue has appeared, it will become too soft even for 

 pen knives. These colors are vivid, and though superficial, 

 are completely indicatory of a change, which can be proved by 

 comparing steel of these colors. See Priestley. 



In this case it would appear that in the course of combustion, 

 a positive combination is formed between the steel and some 

 of the rays of light ; thus, first the yellow ray, then the yelloAV 

 is mingled with red, by a species of oxidation, which gives the 

 further facility to absorption of hydrogen, forming carbon 

 (composed of the three rays), which assumes the predominancy 

 when the Hue ray becomes evident. The like transition of 

 colors was observed by Boyle, on melted lead, in the act of 

 cooling, by each succeeding pellicle assuming a rapid display 

 of colors, but not regular in their succession as the steel. Thus, 

 first, blue yellow and purple, blue green and purple, blue 

 yellow and blue ; red, blue, green, yellow, red, purple, and 

 whatever color was displayed in the act of setting or cooling 

 below fluidity, the metal retained superficially. 



Now in the oxidation of lead, it would appear that until 

 brought to a certain state of combination with the oxygen, the 

 other rays of light are partially combined with the lead, and 

 give evidence of their presence, but are ultimately driven off 

 by its affinity for the oxyginating ray. 



Kircher and Boyle observe on the lignum nephuticum of 

 Mexico, that the infusion is limpid in strong light, in light shade 

 is a beautiful green, more shaded, inclined to red, in a very shady 

 place, and in an opaque vessel, green again. Physiologia 57. 

 In transmitted light, a golden color ; in reflected light, ap- 

 pears a lovely blue color ; shadow projected on it, tinged with 



