114 Prof. C. Bolm on Negative Fluorescence 



attained very quickly; and it appears, from observations to be 

 communicated hereafter, that this maximum is not reached in 

 5J minutes (the time which elapsed, in the experiment recorded 

 above, before phosphorescence began, while in other cases it oc- 

 curred still more quickly) . The radiation from the glass pro- 

 ceeds, however, from greater proximity than that from the walls 

 of the stove. But it is impossible to obtain in this way any cer- 

 tain estimate of the quantitative ratio of the radiation upon the 

 crystal in the glass cell as compared with that upon the unco- 

 vered crystal. It might therefore be more prudent not to insist 

 upon the retardation of the phosphorescence in the glass cell as 

 disproving the direct transformation of rays ; but this retarda- 

 tion certainly does not speak in favour of ray-transformation. 

 I may mention further that when the fire in the stove was kept 

 so low that phosphorescence occurred slowly and with difficulty 

 in the uncovered crystal and in that in the salt cell, it did not 

 t)ccur at all in the crystal in the glass cell, even when the latter 

 had a more favourable position in the heated space than the 

 others. 



6. The elevation of temperature in the crystals of fluor-spar 

 is owing to absorption of the heat radiated directly by the sides 

 of the stove*, to contact with the continually renewed heated 

 air, and, lastly, in the case of the pieces hanging inside the cells, 

 to absorption of the heat radiated by the sides of the envelope. 

 As the air can pass in and out of the cells only by a few small 

 openings, it cannot change so easily and quickly as the air sur- 

 rounding the uncovered crystal. The heating by conduction 

 must therefore take place more slowly in the case of the crystals 

 enclosed in the cells than in that of the one suspended in the 

 open air. On the other hand, the heating by absorption of radiant 

 heat must take place very nearly at the same rate in the case of this 

 last crystal as in that of the crystal in the salt cell. If the lumino- 

 sity is solely an effect of temperature, and independent of any di- 

 rect influence of the radiation to which the crystals are exposed, 

 the slower heating of the crystal in the salt cell explains why it 

 is longer in becoming phosphorescent than the freely suspended 

 piece. The heating due to contact with the hot air goes on at 

 the same rate in the case of the equal-sized pieces placed in en- 

 velopes of the same size and shape ; but the heating due to ab- 

 sorption is more rapid for the crystal in the salt cell than for 

 that in the glass cell, inasmuch as the sides of the latter become 

 heated and begin to radiate only gradually, whereas the rays 

 emitted by the sides of the stove can pass through the plates of 

 rock-salt and thus act directly upon the mineral. Hence the 



* According to Melloni's experiments, a plate of fluor-spar absorbs 

 about 60 per cent, of the invisible rays of heat. 



