142 THE PHOSPHORESCENCE OF BODIES. [MKMOIR VIII. 



substance should be controlled by these conditions, and 

 fluor-spar completely fulfils all the indications. 



It will be seen, however, that I have not restricted 

 myself to the use of this body, but whenever other sub- 

 stances could be compared with it, have resorted to them 

 also. The general principles here set forth as applicable 

 to fluor-spar may be likewise extended to them. 



To what cause are we to attribute phosphorescence? 

 What are the changes taking place in the glowing 

 body? 



We have already seen that a century ago two different 

 answers had been given to these questions. Lemery 

 supposed that all bodies act towards light as they do to 

 heat, absorbing it and then giving it out : Du Fay that 

 all phosphorescences are cases of combustion. 



Before we can reach a decision there are evidently 

 many preliminary points to be settled. If -chemical 

 changes between the glowing body and the air are dis- 

 posed of, and the action is recognized to be of a purely 

 physical or molecular kind, it is necessary to determine 

 (1) whether there is any expansion or contraction of the 

 shining body during its glow ; (2) whether there is any 

 structural change; (3) whether there is any evolution 

 of heat along with the light ; or (4) any development of 

 electricity. These inquiries will now be taken up in 

 succession. 



I. Is there any change of volume in a phosphorescent 

 body during its glow f 



I attempted to ascertain this by causing various bodies 

 to shine brightly when enclosed in glass vessels filled 

 with water, so that if there were any expansion the 

 water might be pressed out into a slender tube, and the 



