206 WRITINGS OP JOSEPH HENRY. [1843 



with the path of the spark. After the discharge, the sulph- 

 uret of lime under the opening was observed to be marked 

 with a narrow line of light well defined at its edges, and 

 shaded off at its ends into a penumbra ; the appearance be- 

 ing precisely in accordance with the laws of a radiation in 

 straight lines from a narrow line of emanation. 



Experiments were next made to determine whether the 

 radiation of the emanation takes place with the same inten- 

 sity from every point of the length of the spark, or whether 

 it is confined to the two extremities, or the poles of the dis- 

 charging wires. For this purpose the slits were turned at 

 right angles to their former position, so that the emanation 

 could onl}'^ reach the lime from a single point of the spark. 

 The experiments with this arrangement showed that the 

 radiation is from each point of the line of the spark, but 

 that it is much more intense from the two extremities. 

 This curious result was verified by another arrangement, 

 which allowed the impressions from difi'erent points of the 

 spark to be at once compared with each other. Three slips 

 were cut in a thick plate of mica, and this was placed im- 

 mediately above the line, so that one of the slits was directly 

 under the end of each wire, and the other midway between 

 the other two. When the discharge was passed over the 

 plate, the lime under the middle slit exhibited a feeble 

 phosphorescence for two or three seconds, and then became 

 dark, while that under the slits at the end of the spark con- 

 tinued to glow for more than a minute. This efiect did not 

 appear to be due to the difi'usion of the spark at the middle 

 of its course, since the discharge was from a Leyden jar, and 

 the spark as is usual in this case, appeared as a single line 

 of light, of the same intensity and width throughout its 

 whole length. 



The phosphorescence was excited at a much greater dis- 

 tance than was at first thought possible. In a perfectly dark 

 room, the light was observed for a few moments when the 

 pan containing the lime was removed to the distance of ten 

 feet from the point of discharge. The intensity of the light 

 and the time of continuance however diminished very 

 rapidly with an increase of distance. 



