33 



LIGHT. 



Under Optics, the mechanical properties of light 

 were considered. 



Light has considerable influence on chemical 

 operations, but little is known of its real nature. 

 Most generally it is considered as a certain simple 

 substance, of which the chief source is the sun ; 

 and it is also disengaged during the processes of 

 combustion. 



The most delicate experiments have been insti- 

 tuted for the purpose of discovering whether it 

 has weight, but without success ; on which account 

 it is reckoned among the imponderable bodies. - 



There appears to be an intimate connexion be- 

 tween light and heat, and they are frequently 

 given out together. But although they are both 

 always found in the sun's rays, yet from them 

 they may be obtained separately, the iusisible rays 

 of heat being more refrangibible than those of 

 light : see vol. i. 



Light is capable of entering in union with many 

 substances, and of being again separated from 

 them. This is the case in the substance called 

 pyrophorus, which is made by exposing to a red 

 heat in a crucible for some time, a mixture of 

 pounded oyster shells and sulphur. If this sub- 

 stance be then carried into the light for a few 

 seconds, it will imbibe so much that it will become 

 luminous in the dark by again giving out this light. 



Various kinds of meat, but particularly fish 

 when they are beginning to putrefy, also rotten 

 wood, sea-weeds, and some insects, as the glow- 

 worm and lanthorn fly, have the property of shining 

 in the dark. 



VOL. II. D 



