CusACK — Temperature — Phosphorescent Substances. 535 



bottle was then filled with water. Now when the stopper was put in, 

 the water rose in the capillary tube to a certain height. The bottle 

 and its contents were then illuminated by the light from the spark 

 caused by the rapid discharging of a Leyden jar connected with the 

 terminals of an induction coil. If the water rose in the capillary 

 tube, as I found it did, either something must have expanded, or 

 the bottle contracted ; the expansion, however, was observed whether 

 a phosphorescent substance was in the bottle or not, and I sub- 

 sequently found that the expansion was due solely to heat. This 

 apparatus was therefore abandoned, and the following, " IM'ewton's 

 King '' apparatus, adopted instead : — 



This instrument consists of three brass pillars parallel to each 

 other, secured into a heavy metal base. On each of these pillars is 

 cut a screw of forty threads to the inch, reaching over the upper six 

 inches of the pillars which are eight inches high. A flat ring of brass, 

 with three holes cut in it for the pillars to pass through, is supported 

 by nuts which run on the screws ; this ring can thus be placed at 

 any height on the pillars that is necessary. A convex mirror and a 

 lens suitable for producing interference rings by reflection are con- 

 tained in a circular brass box, which box fits into the centre of the 

 ring. The whole lens apparatus can thus be raised or lowered as is 

 necessary without disturbing the lens ; also dust is not so liable to 

 get between the lens and mirror. The lens is stationary, being 

 cemented to the case, and the mirror is supported on the top of a 

 screw, by means of which it can be adjusted so as to get the rings in 

 a suitable position. Through this screw passes a steel rod, on the 

 upper end of which the mirror when adjusted rests, the lower end 

 resting on the crystal, or other substance under observation. The 

 whole apparatus is then surrounded by a cardboard cylinder, and by 

 adjusting a slit in this cylinder a beam of light can be arranged so as 

 to strike the substance under observation only. The light used was that 

 from the spark of a Leyden jar as previously mentioned. "When a 

 crystal of fluorspar was taken, and as much as possible of the light it 

 contained driven out without cracking the crystal, it was adjusted, 

 sitting on the bed-plate and having the steel rod resting on the upper 

 edge of a face ; the whole length, therefore, of one face of the crystal 

 was between the bed-plate and the rod which supported the mirror. 

 The apparatus, when adjusted, was so delicate that the heat of the 

 fingers when adjusting expanded the supports, which were allowed 

 to cool before an experiment was attempted. Sodium light was 

 used to illuminate the rings, and when the cross wires of the 



