112 Prof. C. Boh n on Negative Fluorescence 



of them made of live plane-parallel plates of equal thickness, and 

 formed three-sided prisms with square far-projecting bases. 

 The three vertical sides were cemented on to the lower base- 

 plate ; a piece of a knitting-needle was placed at right angles 

 across their upper ends, and from this the fragment of the crys- 

 tal was hung by a thin cotton thread so that it did not touch 

 the sides of the cell. The upper base-plate was laid on loose. 

 Small holes were made above and below in the vertical sides so 

 that the air could circulate through the cell; but the far-project- 

 ing terminal plates made it impossible for rays of heat ema- 

 nating from the walls of the oven to reach the crystal inside 

 without penetrating the solid substance of the envelope. The 

 cells were supported on low tripods of thin iron wire, and the 

 first of the three pieces of fluor-spar was hung from a support 

 made of iron wire by a cotton thread of such length that it was 

 in the same horizontal plane with the two pieces of crystal in 

 the cells. The glass cell with its contents was first put in its place 

 in the heated space, then immediately afterwards the salt ceil, 

 and lastly the piece of fluor-spar which was suspended freely in 

 the air. I selected positions in the heated space for which the 

 conditions as to radiation &c. were as little unequal as possi- 

 ble; but as small inequalities were unavoidable, the places of 

 the several apparatus were interchanged in the various experi- 

 ments. The several pieces of crystal were also interchanged, so 

 that the one which had previously been in the salt cell came 

 to be in the glass cell or in the free air, and vice versa. When 

 the first apparatus was put in its place, the time was noted 

 by the clock, and the seconds were counted to ascertain how 

 much later each of the others came into its place. When the 

 arrangement of the apparatus was completed, all the light in the 

 room was extinguished, and the first beginning of luminosity 

 was carefully watched for in the perfectly dark space. The time 

 that elapsed between putting each piece of crystal in its place 

 and the commencement of luminosity could be measured by a 

 striking seconds-clock. I never allowed the pieces of fluor-spar 

 which were to be used afterwards for further experiments to be 

 phosphorescent for more than two minutes at the outside ; and 

 I endeavoured in most cases to remove them from the heated 

 space as soon as possible after the luminosity was unmistakeably 

 perceived. 



4. If the crystals and the cells had the temperature of the 

 room, namely about 18° C, before they were put into the heated 

 space, the piece which was suspended without covering became 

 luminous first, then considerably later the piece in the salt cell, 

 and lastly, after a long additional interval, the piece in the glass 

 cell. Thus, for example, in one experiment (in which the stove 



