on the Existence of a Fourth Imponderable. 113 



the voltaic deflagration of any metallic bodies, the observation 

 applies to transient discharges only. A voltaic light, which 

 lasts but a moment, fails to cause phosphorescence through 

 glass in the same way that an electric spark does ; but if the 

 discharge is continued, the surface presently begins to glow ; 

 and if maintained for several minutes, it shines as brightly as 

 though a piece of quartz had been used. 



The inability of an electric spark to cause phosphorescence 

 is connected with its transient duration. The voltaic light 

 enables us at pleasure to imitate the eiFects of an electric spark 

 or those of the sun. 



The phosphorogenic rays, whether they originate in an 

 electric spark or from the sun, occupying thus the same place 

 in the spectrum, and even exhibiting the same peculiarities as 

 the tithonic rays on iodide of silver, we have next to deter- 

 mine whether this is an apparent or a positive identity. 



Prof. Henry of Princeton read a paper before the Ame- 

 rican Philosophical Society in May 1843, in which he dis- 

 cussed all the leading mechanical properties of the phospho- 

 rogenic rays, and aiivong other important experiments made 

 some with a view of determining this particular question. A 

 Daguerreotype plate and some sulphuret of lime were simul- 

 taneously exposed to the sky ; the plate was stained, but no 

 effect was produced on the lime. A Daguerreotype plate and 

 some sulphuret of lime were exposed to the light of an electric 

 spark ; the lime was observed to glow, but no impression was 

 produced on the plate. When the plate was exposed to a 

 succession of sparks for ten minutes, with a sheet of mica in- 

 terposed, an impression was made. Lime exposed to the moon 

 did not phosphoresce, but a sensitive plate under the circum- 

 stances is said to be stained. In view of these different facts, 

 Prof. H. observes, " These experiments, although not suffi- 

 ciently extensive, appear to indicate that the phosphorogenic 

 emanation is distinct from the chemical, and that it exists in 

 a much greater quantity in the electric spark than either the 

 luminous or chemical radiation." 



From Wilson's experiments, it appears that he was aware 

 that when the phosphorescent surface is warmed, so as to 

 hasten the disengagement of light, the moonbeams may be 

 found to have left traces of action upon it ; feeble it is true, 

 but nevertheless very apparent. We have seen also that the 

 peculiarity of an electric spark is due to its transient duration. 

 Before therefore a final decision can be obtained on this point, 

 we are required to examine the effect of the tithonic rays and 

 phosphorogenic emanation, under circumstances which are 

 precisely similar as to intensity and time. 



Phil Mag, S. 3. Vol. 25. No. 164.. August 1844. I 



