204 WRITINGS OF JOSEPH HENRY. [1843 



EXPERIMENTS ON PHOSPHORESCENCE. 

 (Proceedings of the American Philosophical Society, vol. iii, page 38-44.)* 



May 26, 1843. 



Professor Henry presented a communication " On Phos- 

 phorogenic Emanation," and illustrated by numerous dia- 

 grams the experiments which he had made on the subject. 



It has long been known, that when the diamond is ex- 

 posed to the direct light of the sun, and then removed to a 

 dark place, it shines with a pale bluish light, which has 

 received the name of phosphorescence. The effect is not 

 peculiar to the diamond, but is common to a long list of 

 substances, among which the sulphuret of lime (Homberg's 

 phosphorus) is the most prominent. It is also an old fact, 

 mentioned by Canton, that the phosphorescence is excited 

 by exposing the substance to the light of the electrical dis- 

 charge. 



About three years ago, M. Becquerel, of the French Insti- 

 tute, repeated the experiment of Canton, and discovered the 

 remarkable fact, that the phosphorescence is excited in a 

 very feeble degree, or not at all, when a plate of glass or mica 

 is interposed between the spark and the sulphuret of lime, 

 although the effect is not apparently diminished when a 

 plate of rock crystdl or one of sulphate of lime is similarly 

 interposed. Or in other words he found that substances 

 equally transparent do not equally well transmit the excit- 

 ing cause of the phosphorescence. Hence the old explana- 

 tion of the glowing of the diamond, namely, that it is owing 

 to the light which has been absorbed and is again given off 

 in the dark, could no longer be admitted ; and Becquerel 

 inferred from his experiments, that the exciting cause of the 



* [The title-page of vol. iii, (comprising the proceedings only from May 

 25 to May 30,) bears date 1843. The proceedings occupying a series of special 

 meetings held during the mornings and evenings, were appointed to com- 

 memorate the hundredth anniversary of the American Philosophical Society: 

 and the volume containing them was published in advance of the preceding 

 volume II.] 



