144 TIIE PHOSPHORESCENCE OF BODIES. [MEMOIR VIII. 



On making the trial, and using in succession a crystal 

 of violet-colored fluor-spar, a piece of flesh-colored chloro- 

 phane, and a mass of Canton's phosphorus, the result in 

 all cases was negative ; for, though these different sub- 

 stances glowed very brilliantly as soon as the spark 

 passed, there was not the smallest movement perceptible 

 in the index liquid of the thermometer tube. 



With a view of estimating the delicacy of the means 

 thus used for determining any change in the volume of 

 the spar, the solid content of a piece of chlorophane was 

 determined by weighing in water; also the value of each 

 division of the scale was ascertained. The value of each 

 such division was equal to -j^Vo of the volume of the 

 spar, and a movement equal to one tenth of that value 

 could have been detected. 



It may therefore be concluded that a phosphorescent 

 body, when at its maximum of glow, has not changed its 

 volume perceptibly. 



The conclusion thus arrived at is strengthened by an- 

 other mode of experiment. If change of volume be con- 

 nected with this evolution of light, it might reasonably 

 be expected that a sudden, severe, but equable compres- 

 sion, exerted on a piece of spar, the light of which is just 

 fading out, would compel it to regain a portion of its 

 brilliancy. A piece of chlorophane in that condition was 

 placed in water contained in the apparatus known as 

 Oersted's instrument for measuring the compressibility 

 of water, and which is described in most of the treatises 

 on physics ; but though, by suitably turning the screw, 

 pressures varying from one to four atmospheres were 

 suddenly put on the spar and as suddenly removed, no 

 change whatever was seen in the glowing mass, the light 

 of which continued steadily to die away. 



In Fig. 15, Oersted's instrument for proving the com- 

 pressibility of water, a a is the glass cylinder filled with 



