536 Proceedings of the Royal Irish Academy. 



telescope used for reading was adjusted, a movement of tlie eighth of 

 a wave-length of sodium light could he easily read. Now with a 

 pillar of fluorspar over three inches long no movement of the ring 

 was ohserved ; that is to say, the crystal did not expand the one four 

 hundred and fifty thousandth of its length. I also tried pillars of 

 calcium sulphide without any result. 



It was while making the foregoing experiments that I ohserved 

 that a piece of fluorspar when heated for a second time gave out light, 

 though when the first heating had ceased no light was heing given 

 out. 



It was found when the matter was looked into that at the second 

 heating the temperature was higher than it had heen in the first. 

 The following are accounts of the experiments made : — 



A crystal of fluorspar was broken up into pieces about the size of 

 a nut ; some of these pieces were taken and heated in a sand bath for 

 four hours, the temperature being kept as nearly as possible at 300° C. 

 The spar glowed for about four hours, after which time the glow 

 ceased. The crucible was then allowed to cool till next day. Now 

 when these pieces of spar were heated again they did not glow till the 

 temperature was raised above the temperature of 300° C. ; to which 

 they had been raised when first heated. The glow off the spar when 

 heated for the second time was easily observed, though rather faint. 

 "When the spar was heated for the second time it was raised to about 

 500° C, and glowed for nearly an hour. These temperatures were 

 obtained by observing when certain minerals, whose melting points 

 were known, fused. The temperature could be kept fairly constant, 

 as the experiments were conducted in a room without di'aughts, heat 

 being supplied by a Bunsen burner with the flame kept to a certain 

 height, also a large quantity of sand was used in the crucible. By 

 making further experiments at other temperatures I found that 

 fluorspar always contained some light until it was heated to a tem- 

 perature of about 500° C. ; at which temperature all the light was 

 expelled. Fluorspar raised to this temperature, allowed to cool, 

 and again lieated, omitted no light, no matter to what temperature 

 it was subsequently raised. 



It was found tliat fluorspar might be kept at any temperatui'e up 

 to about 500° C. for a long time without losing all its light. In fact 

 I kept some pieces of spar for two days at about 300° C, yet these 

 pieces glowed when afterwards raised in temperature. 



I now exposed some fluorspars at 300° C. to the sparks from a 

 Leyden jar coupled with the poles of an induction coil, and found that 



