MEMOIR VIIL] THE PHOSPHORESCENCE OF BODIES. 143 



amount of dilatation thereby determined. The arrange- 

 ment was as follows : 



A glass tube about two inches long and three quar- 

 ters of an inch in diameter was closed at its upper end 

 by means of a plate of polished quartz, cemented air- 

 tight. Immediately beneath the quartz the phosphores- 

 cent body was supported. Through a cork which closed 

 the other end of the tube there passed a piece of ther- 

 mometer tube bent on one side, and to it was affixed a 

 scale. The arrangement was supported on a suitable 

 stand, so that the quartz was uppermost, and at a little 

 distance above it the spark from a Leyden-jar could be 

 passed between a pair of stout iron wires maintained at 

 an invariable distance, and thus produced phosphores- 

 cence in the body. It may be remarked that these ef- 

 fects of an electric spark do not take place well through 

 glass, and hence a plate of quartz, which readily trans- 

 mits them, must be used. 



In Fig. 14, a a is the glass tube, b b the plate of pol- 

 ished quartz, c the phosphorescent body, 

 d d the cork closing the lower end of the 

 tube, e e the bent tube,/ its scale, g g the 

 iron wires connected with a Leyden-jar, 

 and giving a spark. The index drop at 

 li refers not to this, but to a subsequent 

 experiment. 



The large tube containing the phosphor- 

 escent body must be filled quite 

 full of water, free from air, as also 

 f must be the thermometer tube to 



Fig. i4. a given mark on its scale. If an 



electric spark be now passed between the wires to make 

 the phosphorus shine, it is clear that if there be any ex- 

 pansion or contraction of its volume, there will be a corre- 

 sponding movement in the water of the thermometer tube. 



