The Nineteenth Century 197 



in combination with those bodies at a higher temperature than that of 

 our atmosphere; at the common temperature it is liberated. . . . 



Other bodies exist, which become luminous when their repulsive 

 motion is increased by communication of it from some bodies of a higher 

 temperature. Light remains in combination with these bodies only at a 

 low temperature. When their repulsive motion is increased, the light is 

 liberated. This decomposition appears to arise from the diminution of 

 the chemical attraction between light and the body, by the repulsive 

 motion, and from the supply of a quantity of it sufficient to enable light 

 to fly off in the repulsive projectile form. Amongst these bodies are the 

 different combinations of lime, and particularly the fluate,^ (the colours 

 of which appear to depend upon combined light,) different combinations 

 of barytes, the sulphate of potash, some of the metallic oxyds, cotton, 

 wool, oils, wax, alcohol, 8cc. . . . 



There is a class of phosphorescent bodies, which give out their com- 

 bined light on attrition. Amongst these are borate of soda, sulphate of 

 argil, ^ tartrite of potash, and all the silicious class of stones. This phos- 

 phorescence may be accounted for in the same manner as the last species. 



Certain substances give out their combined light on immersion into 

 the mineral acids. When magnesia is thrown into the sulphuric acid, a 

 light is liberated which produces a sensation similar to that known by 

 the name of red heat. The same effect is produced when the nitric acid 

 is used. 



During the combination of lime with the mineral acids, a flash of 

 white light is uniformly perceived; the same effect is not produced during 

 the combination of strontian and barytes with these acids. 



This phaenomenon appears to be owing both to the attraction of the 

 acids, and to the repulsive motion generated during the combination, a 

 motion sufficient to give to the combined light repulsive projection; for 

 lime and magnesia become luminous when heated, which is not the case 

 with strontian and barytes. 



Finally in Elements of Chemical Philosophy (1812) [Opera 4: 

 163) , in speaking of " Radiant or Etherial Matter " Davy wrote: 



Many phenomena which have been attributed to combined light, 

 appear to be electrical, or to be merely the effect of the ignition of the 

 substances, for whenever heat rises beyond a certain degree, bodies be- 

 come luminous; pieces of quartz rubbed together are rendered electrical; 

 and by percussion or friction any hard bodies may be intensely heated. 



During the putrefaction of certain animal and vegetable substances, 

 light is emitted; and this is no more difficult to account for, than the 

 heat produced during similar operations. 



The light emitted by certain living insects, appears to depend upon 

 the secretion of a substance very easy of decomposition: and any chemi- 

 cal change may be supposed adequate to the production of light. 



^ Fluorspar or calcium fluoride. 



* Argil is a term for potter's clay, but may also refer to aluminium. 



