88 Prof. Draper on the Phosphorescence of Bodies. 



A flat piece of fluor-spar, polished on both sides, was placed 

 in a polariscopc, and a pair of blunt iron mres connected with a 

 Leyden jar were adjusted very near and in front of it ; so that 

 when the spark passed, a brilliant glow ensued in the spar, which 

 was simultaneously viewed through the analyser of the instru- 

 ment. But though the experiments were made both by daylight 

 and lamplight, no kind of effect could l)e detected. 



These experiments were first made by using as the analyser, 

 a doubly refracting achromatic prism ; they were, however, re- 

 peated with a Nichol's prism, in which the eye is not disturbed 

 by a bright image, as in the other case. Having fixed the plate 

 of polished spar in the polariscopc, it was readily perceived that 

 it possessed naturally a kind of structm-al arrangement, because 

 there were cloudy spaces or lines in it which contrasted with the 

 faint white light which passed in the adjacent parts. It was 

 also seen that this structm-al arrangement could be deranged, in 

 a transient manner, either by pressure or unequal warming, as 

 is well known of other bodies ; but when a powerful electric 

 discharge was passed near to the spar and a brilliant phospho- 

 rescence took place, no change whatever could be discovered. 

 Even when the iron mres were made to rest on the spar and 

 the explosion passed over its siirface, nothing was perceptible 

 except along the line between the ends of the wires, where the 

 surface was roughened or abraded by the force of the discharge. 



But though these experiments with polarized light give a ne- 

 gative result, or at all events prove that a phosphorus, when 

 glowing, has its molecular arrangement so little disturbed that 

 the change cannot be detected in this way, it appears to me there 

 can be no doubt that if om- means of testing were more delicate, 

 such a change woiUd Ije discovered ; for many years ago, IMr. 

 Pearsall found that specimens of fluor not possessing phospho- 

 rescence naturally, might have that quality communicated to 

 them by repeated exposure to many powerful electric discharges. 

 In those cases, it was also seeia that a change of colour was gi-a- 

 dually brought about in the specimen by the discharges. 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 ai'C merely supei-ficial ; it is the mode in which various 

 vapours will condense. I described several such cases in the 

 Philosophical IMagazine for September 1840 ; considerable atten- 

 tion was subsequently given to them by M. Moser. They were 

 introduced in that memoir as an illustration ofthe mode in which 

 mercmial vapours condense on the Daguerreotype plate, — an 

 illustration which I still believe to hold good. 



Proceeding on this principle, a large plate of fluor-spar, the 



