LIGHT. 29 



14. Light is emitted as well as absorbed by bodies. Bodies that 

 emit light are called phosphorescent ; heat does not accompany this 

 luminous emission. 



(a.) Solar phosphori are those which after exposure to the sun, for 

 some time, emit light in the dark. Du Fay having exposed a diamond 

 to the sun and immediately covered it with black wax, it shone in the 

 dark at the end of several months, when the wax was removed. 



"In 1663, Mr. Boyle observed that the diamond when slightly 

 heated, rubbed, or compressed, emitted a light almost equal to that 

 of the glow worm." Ure. 



Snow has been supposed to be a natural solar phosphorus, but this 

 appears to be incorrect ; for it does not shine in a perfectly dark 

 place ; it seems to operate merely by reflecting the light which is 

 abroad even in the night, except when the clouds are very heavy, in 

 the absence of the moon. 



(b.) There are artificial solar phosphori. 



Canton's preparation. Sulphuret of lime, made by stratifying 

 burnt oyster shells and flowers of sulphur, and heating them in a phial, 

 or in a crucible in a furnace. 



Bolognian phosphorus, viz, sulphate of barytes partially decom- 

 posed into a sulphuret by ignition, with flour, sugar, gum arabic. 

 starch, &c. 



Baldwin's phosphorus is fused muriate of lime. Homberg's de- 

 pends on combustion. (See alum.) 



Herring, mackarel, (or other marine fish,) being put into a phial 

 ivith water and about one eighth of its weight of common, Epsom, or 

 Glauber's salt, and conveyed into a dark place ; a luminous ring is seen 

 after three days, and the whole fluid appears luminous when agitated.* 



The phosphorescence of fish when hung up in a chimney corner, 

 and of rotten wood, &ic. is probably owing to decomposition prece- 

 ding putrefaction. Peat earth is phosphorescent. 



Canton's preparation and other solar phosphori, on being exposed 

 to the light, shine in the dark, so that we may tell the hour by a 

 watch, and when they cease to shine, they again acquire the power 

 by a new exposure. 



(c.) Some bodies become phosphorescent by heat. Fluor spar, 

 phosphate of lime, many varieties of feldspar, and many lime stones 

 are of this class. It is usual to pulverize them coarsely, and to throw 

 them upon a red hot shovel in a dark place. The fluor spar from 

 Monroe, seventeen miles west from New Haven, is a most remarkable 



* If the saline solutions are too strong they do not shine, but the light instantly 

 appears on dilution with water. Ebullition destroys, but congelation only suspend* 

 <he* property, which appears again on thawing. Ure. 



