Prof. Draper on the Phosphorescence of Bodies. 87 



in water ; also the value of each division of the scale was ascer- 

 tained. The content of the thermometer-tube employed, for 

 each division of its scale, was equal to yg^-dth of the volume of 

 the spar, and I could have detected a movement equal to one- 

 tenth of that value ; so that had there been any increase or dimi- 

 nution in volume by that small amount, the effect would have 

 been apparent. There was, however, absolutely no movement 

 perceptible. Hence it seems that — 



A phosphorescent body, when at its maximum of glow, has not 

 changed its volume perceptibly . 



The conclusion here arrived at is strengthened by another 

 mode of inquiry. If change of volume is connected with this 

 evolution of light, it might reasonably be expected that a sudden, 

 severe, but equable compression exerted on a piece of spar, the 

 light of which is just fading out, would compel it to regain a 

 part of its brilliancy. A piece of chlorophane in that condition 

 was placed in water contained in the apparatus kno^vn as (Ersted's 

 machine for proving the compressibility of water, and which is 

 described in most of the treatises on physics ; but though, by 

 suitably turning the screw, pressm-es vai-ying from one to fom* 

 atmospheres were suddenly put on the spar, and as suddenly 

 removed, no change whatever was seen in the glowing mass, the 

 light of which steadily continued to die away. 



In passing it may be remarked, that these experiments seem 

 to have a bearing on Lemery's theory. A mass of iron, suddenly 

 compressed, grows hot ; so, too, does atmospheiic air. It would 

 not therefore be unreasonable to expect, that if a phosphorus 

 acted like a sponge towards light, if it be thus pressed upon, it 

 would let its light go. But the relations of specific heat are 

 perhaps scai-cely applicable here. 



Wliere unequal pressure is applied the result is different. A 

 piece of chlorophane, pressed by forceps, glows brightly; if 

 crushed, the fragments sparkle like little fireworks as they fly 

 tlirough the air. If the spar be previously powdered, a shining 

 is still produced; and when the pulverization is conducted in 

 an agate mortar in the dark, bright eddies of light follow the 

 track of the pestle. In these cases, however, the separation of 

 the laminae of the crystal, and the heat ])roduced by friction, 

 probably determine the result. Canton's phosphorus did not 

 appear to shine when compressed or submitted to friction. 



2nd. Does any structural ctiange accompany the phosphorescence 

 of bodies 'i 



The foregoing experiments appearing to prove, that if there 

 be any expansion of a phosphorescing body it is to a very small 

 amount, I next proceeded to determine whetlier there is any 

 molecular change, or new structural arrangement, which can be 

 detected by polarized light. 



