Light. 139 



lognian stone in its property of imbibing light, 

 and emitting it after it was brought into the 

 dark, whence it has been termed Baldwin's 

 phosphorus. In truth, .the same effect may be 

 produced from calcined oyster-shells, and from 

 all the varieties of that mineral called ponderous 

 spar, of which the Bolognian phosphorus is a 

 species. Diamonds ateo, and some emeralds, 

 and other precious stones, will emit light when 

 carried out of^a light into a dark place. The 

 light emitted by these phosphor! always bears 

 an analogy to that which they have imbibed. In 

 general it is reddish ; but when a weak light only 

 has been admitted to them, or when it has been 

 received through white paper, the light which 

 they give out is pale or whitish. 



III. Notwithstanding the rarity of light, how- 

 ever, and the smallness of its particles, it is not 

 destitute of force or momentum. To prove this, 

 a most ingenious experiment was made by the 

 late Mr. Mitchell. He constructed a small vane 

 in the form of a common weathercock, of a very 

 thin plate of copper, about an inch square, and 

 attached to one of the finest harpsichord wires, 

 about ten inches long, and nicely balanced at 

 the other end of the wire by a grain of very 

 small shot. The vane was supported in the 

 manner of the needle in the common mariner's 

 compass, so that it could turn with the greatest 

 ease; and to prevent its being affected by the 

 vibrations of the air, it was enclosed in a glass 



