OusACK — Temperature — Phosphorescent Substances. 541 



before the calcium sulphide (a) had reached its maximum ; also that 

 calcium sulphide seems to have a larger range of temperature over 

 which it phosphoresces than strontium sulphide. The specimen of 

 barium sulphide (b) has not reached its maximum at - 80°, when the 

 other two substances have greatly diminished in brightness. 



The following are the results of experiments from which the 

 curves (a) for calcium sulphide were plotted : — 





No. of 

 Papers. 



No. V 15. 



Antilog of 

 No. + 15 



+ 283° 



13-0 



•86 



73-6 



230° 



9-0 



•60 



39^8 



194° 



6-8 



•453 



28-4 



139° 



6-3 



•42 



26-4 



96° 



7-0 



•46 



29-3 



49° 



7-5 



•50 



1-6 



+ 16° 



8-0 



•53 



34-0 



-40° 



10-0 



•66 



46-4 



-80° 



11-3 



•753 



56-7 



The number of papers was divided by 15 to make the curve 

 plottablCj and the antilogs there given have been multiplied by 10 

 for the same reason. From the antilogs given here a curve was 

 plotted, and from this curve once obtained, the reciprocal curve was 

 afterwards plotted ; this was done in the case of each substance. 



The great difficulty in making observations is working the 

 photometer, and on account of the errors arising from this source 

 these curves are only approximate. I have tested them in several 

 ways. Thus when a curve is drawn, by making an observation and 

 seeing if it agrees with the curve, we have an excellent test, and I 

 have found the observation falls usually a trifle too low. A difference 

 of one paper interposed makes a very considerable difference in the 

 plotted curve, and it is very hard to judge to less than one paper. 



With reference to the curves, a is that for calcium sulphide ; b for 

 barium sulphide, and c for strontium sulphide ; the corresponding small 

 letters indicate the antilog curves from which the intensity curves were 

 plotted. 



