210 WRITINGS OP JOSEPH HENRY. [1843 



quently slow restoration of the equilibrium. The result 

 however of the following experiment would seem to be at 

 variance with this explanation. The lime was thrown into 

 a tumbler of water, and sank to the bottom, but in this situa- 

 tion, when the spark was passed over the surface of the 

 liquid, it became as luminous and the effect appeared to 

 remain as long as when the exposure took place in the air. 



The author stated that some of the experiments described 

 by him can be repeated with common chalk, although it is 

 not as sensitive as the sulphuret of lime. Some pieces of it 

 however become luminous at a considerable distance, and it 

 is not improbable that the chalk cliffs of England are some- 

 times rendered phosphorescent by flashes of lightning during 

 a thunder storm. 



But the substance which gives the most brilliant light, 

 although the light does not continue so long, and is not as 

 easily excited as that from the lime, is the sulphate of potassa. 

 When exposed to the discharge of ajar highly charged, at 

 the distance of a few inches below the spark, it glows for a 

 few seconds with a beautiful azure light; and as this salt is 

 not readily acted on by liquids, it was used to determine the 

 permeability of different substances, by placing a crystal of 

 the salt in the liquid to be tested. 



It has long been known that the sulphate of potassa often 

 emits flashes of light during the progress of its crystallization ; 

 and it is probable that other substances, which are known 

 to emit light under the same circumstances, may also be 

 rendered phosphorescent at a distance by the electrical 

 emanation. 



The following is a list of the substances which have been 

 examined by Professor Henry, with reference to their per- 

 meability by the phosphorogenic emanation : 



TRANSPARENT SOLIDS. 

 Permeable. 



Ice, Sulphate of baryta. 



Sulphate of lime, Sulphate of potassa, 



Quartz, Sulphate of soda. 



