74 CHANGES OF COLOR BY HEAT. 



chanical force requisite to render a pound of iron 

 red-hot. The molecular vibrations of the iron 

 excite vibrations of the electric ether pervading 

 the particles of surrounding air, and transmit the 

 impulses to the retina of the eye, producing the 

 sensation of a red color; and to the nerves of feel- 

 ing, producing a sensation of heat. In suddenly 

 cooling hot steel by the process of tempering, the 

 surface exhibits the changing colors from a white 

 heat to red, orange, purple, violet, and blue ; the 

 latter representing the elastic molecular state of 

 blue watch-springs. These facts indicate that 

 there is a chromatic scale of molecular vibrations 

 developing colors, corresponding with those of the 

 atmospheric and electric ethers developing musi- 

 cal tones. 



The remarkable flitting colors on the surface of 

 heated steel are due to the gradual union of mole- 

 cules of oxygen with the carbonized iron. Thus, 

 metallic oxides serve as pigments, in their peculiar 

 molecular conditions of union with oxygen and 

 carbon. 



MOLECULAR VIBRATION ADOPTED AS A TEST OF 

 HEAT. 



The vibration of molecules of mercury in the 

 bulb of a thermometer, and their consequent oc- 

 cupancy of more space, denoted Expansion, is 

 adopted as a standard test of heat; precisely as 

 the extent of reciprocal divergency of the pith- 



