COMBINATION OF LIGHT WITH STEEL. 81 



8th. That the influence of light, and its combination, in a 

 refrangible state, with fixed, or inert matter, is particularly 

 conspicuous in the growth of vegetable bodies ; and that their 

 health, vigour, inflammability, as well as essential and peculiar 

 qualities, depend on its presence. 



9th. That light also combines, in a similar manner, with the 

 matter of the mineral kingdom; and that its influence is evi- 

 dent on the animal species. 



10th. That if light is admitted to have entered into direct 

 combination with fixed terrestrial matter, it is essential for the 

 economy of nature that it should be again restored to the radi- 

 ant state. 



llth. That from the color of bodies, or that of their solution, 

 in menstrua of known constitution, or of their flame in com- 

 bustion, their leading properties, as well as their original con- 

 stituents, may be ascertained. 



It has been observed by Boyle, and repeated by Priestley, 

 that diversity of colors does not denote any great difference in 

 the internal structure of bodies, yet that it was often an indica- 

 tion of considerable alterations in the disposition of their parts, 

 as appears from the extraction of tinctures, wherever the 

 change of color regulates the artist in the process of their pre- 

 paration : similar instances occur in the maturation of fruits. 

 In like manner, in the method of tempering steel for engravers, 

 drills, springs, &c. First, the steel is hardened by heating it 

 in glowing coals ; it is not quenched as soon as taken from the 

 fire, but held over a basin of water, till it falls in temperature 

 from a white to a cherry-red heat, when it is immediately 

 plunged in cold water. The steel thus hardened looks whitish, 

 and if it is brightened at the end and held in the flame of a 

 candle, so that the bright end projects half an inch distant 

 from the flame, it will swiftly pass from one color to another, 

 as from a bright yellow to a deeper and reddish-yellow ; from 

 thence, first to a fainter, and then a deeper blue, either of which 

 succeeding colors argues such a change made in the texture of 



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