1843] WRITINGS OF JOSEPH HENRY. 205 



phosphorescence was due to an impression made on the lime 

 by a radiation from the electrical spark, differing essentially 

 from light, and to which he gave the name of the "phospho- 

 rogenic emanation." 



Biot afterwards made a series of experiments on the "per- 

 meability " of different substances, in reference to this eman- 

 ation as it exists in the beams of the sun ; and still later, 

 Matteucci, the clebrated Italian experimental philosopher, 

 has investigated and extended the same subject. The younger 

 Becquerel also has published a memoir on the constitution 

 of solar spectrum, including its phosphorogenic properties. 

 From the notices of the labors of these savans on this 

 subject, as they were adverted to, it appears that all their 

 experiments (with the exception of those before mentioned 

 as made by M. Becquerel) were confined to the solar radia- 

 tion, and consequently they do not lessen the importance 

 of a careful examination of the properties of the same ema- 

 nation, as derived from a different source, and having a dif- 

 ferent intensity. 



The investigations detailed in this communication relate 

 almost exclusively to the emanation as derived from the 

 electrical spark. The apparatus employed in the experi- 

 ments was a Leyden jar, of the capacity of about half a gal- 

 lon ; and this was charged each time, so as to give a spark 

 between the rounded ends of two thick wires of about an 

 inch in length. The sulphuret of lime was exposed to the 

 light of the spark at different distances, in shallow leaden 

 pans. The first experiments relate to an examination of a 

 considerable number of substances, in regard to their per- 

 meability by the emanation. The results of these, which 

 were given at the close of the communication, will serve to 

 corroborate the inference of M. Becquerel, that the exciting 

 cause of luminous appearance of the lime is not identical 

 with ordinar}'- light. 



The next experiments are in reference to the propagation 

 of this emanation. Two slits, of about the one-twelfth of an 

 inch wide and an inch long, were made in two screens of 

 sheet brass, and these slits were placed in the same plane 



