1895.] Phosphorescence and Photographic Action. 665 



WEEKLY EVENING MEETING, 



Friday, January 18, 1895. 



Sir Frederick Bramwell, Bart., D.C.L. LL.D. F.E.S., Honorary 

 Secretary and Vice-President, in the Chair. 



Professor Dewar, M.A. LL.D. F.R.S. M.B.I. 



Phosphorescence and Photographic Action at the Temperature of 

 Boiling Liquid Air. 



[Abstract.] 



Continued investigation of the properties of matter at extremely low 

 temperatures has resulted in a considerable addition to our knowledge 

 on this subject, more especially in regard to phosphorescence and 

 photographic action. Phosphorescence and fluorescence are terms 

 applied to similar phenomena which apparently differ only in degree, 

 the first being practically an instantaneous effect, while the other 

 lasts for a relatively long period after the withdrawal of the light 

 stimulus. In all cases the luminous effects called phosphorescence 

 and fluorescence belong to a less refrangible part of the spectrum 

 than the exciting rays. Professor Stokes has shown that the singular 

 surface appearance observed in fluorescent liquids is due to a change of 

 refrangibility of the light absorbed, and again given off by their 

 upper layers. Phosphorescence may be regarded as a kind of 

 fluorescence which lasts a long time after the excitation has ceased, 

 and may be briefly defined as the phenomena observed when certain 

 substances give out light through the transformation of absorbed 

 vibrations of shorter period. This must not be confused with the 

 luminosity due to the slow oxidation of phosphorus ; nor with the 

 " phosphorescent " appearance accompanying the slow combustion of 

 decaying animal and vegetable matter; nor with the more or less 

 voluntary display of light by fireflies, glow-worms and small marine 

 animals. The researches of Becquerel showed that the intensity of 

 phosphorescence depended directly on the product of the intensity of 

 the stimulating light, and a factor of absorption, and inversely, as 

 some coefficient representing molecular friction or damping. When 

 phosphorescing sulphides of calcium are heated they increase in their 

 light emission, whereas if cooled to — 80° they cease altogether to be 

 luminous, and if maintained at this low temperature for hours, keep a 

 latent store of light energy that may be again evolved on allowing the 

 sulphide to rise to the ordinary temperature. 



But while the temperature of— 80° is sufficient to stop all sensible 

 emission from previously-excited sulphide, it does not prevent an 



