MEMOIR VIII.] THE PHOSPHOKESCENCE OF BODIES. 147 



phosphorus when shining has its molecular condition so 

 little disturbed that the change cannot be detected in 

 this way, there can be no doubt that if the means of 

 testing were more delicate, such a change would be dis- 

 covered, for many years ago Mr. Pearsall found that 

 specimens of iluor, not possessing phosphorescence natu- 

 rally, might have that quality communicated to them by 

 repeated exposure to many powerful electric discharges, 

 which also gave rise to a change in their natural color. 

 Now there can be no doubt that such an alteration of 

 tint implies an alteration of structure. 



Besides the test by polarized light, there is another 

 which may be resorted to for the detection of structural 

 changes when they are merely superficial ; it is the 

 mode in which various vapors will condense. I de- 

 scribed several such cases in the Philosophical Magazine 

 for September, 1840, some time previously to the pub- 

 lications on the subject that were made by M. Moser. 

 They were brought forward at that time as an illustra- 

 tion of the manner in which mercurial vapors condense 

 on a daguerreotype plate and develop images which it 

 has received. Proceeding on this principle, a large plate 

 of fluor-spar, the surface of which was finely polished, 

 was made to phosphoresce brightly along a given line 

 determined by the ends of two iron wires, which served 

 as a discharger for a Leyden spark, and were placed 

 near to the polished surface. The spar was forthwith 

 suspended in the mercurial box of a daguerreotype ap- 

 paratus and kept there an hour. The mercury condensed 

 upon it faintly in the manner it would have done on a 

 daguerreotype pi-ate, especially on and in the vicinity 

 of those parts that were more immediately exposed to 

 the spark. This, therefore, seems to prove that there is 

 in these cases a molecular modification of the shining 

 surface. 



