-540 Proceedings of the Royo.l Irish Academy. 



alloys were tried and were fotmd very suitaljle ; an alloy consisting of 

 3 parts of bismuth, 3 parts of lead, and 1 part of tia was used. 

 This latter alloy was melted in a nickel vessel ; ia this a smaller 

 nickel vessel containing the snhstance for observation was immersed 

 and covered np with a piece of cardboard in which there was a hole 

 the same size as that in the photometer. Through this hole the light 

 from the sparks fell on the substance and the light emitted by the 

 latter reaches the eye. A thermometer is also immersed in the merited 

 aUoy. To read this thermometer in the dark a small glow lamp is so 

 arranged as to throw a beam of light along the stem, and can be 

 moved so as to illuminate any part of the column of mercuiy that may 

 be necessary. 



When a phosphorescing substance has been heated and cooled ever 

 so often, so long as it has not been decomposed, the maximum inten- 

 sity of the light emitted at the same temperature is the same, although 

 the time which the substance takes to reach its maximum brightness 

 may vary somewhat ; only two exceptions to this have as yet been 

 observed, and it is doubtful whether these are true exceptions or not, 

 inasmuch as the physical structure of the substance may have altered. 



The fluorides of barium and strontium, after being heated for the 

 first time to a temperature of about 700' C, phosphoresce more bril- 

 liantly than before being heated. But if they are heated again ever 

 so often, they do not appear to phosphoresce more brilliantly than 

 they did after the first heating. Of many other substances examined 

 the only difference observed after being heated was that they 

 reached their maximum brightness sooner than they did before they 

 were heated. The difference in time was however only a second or 

 two. I have failed to observe any difference in the structure of the 

 barium or strontium fluorides under a microscope before and after 

 heating ; but if we suppose that the crystals increase in size when 

 heated, it is obvious that the substance would phosphoresce more 

 brilliantly when the crystals were larger. As the crystals were very 

 minute, not distinguishable imder the microscope, it is quite possible 

 for them to increase in size without the increase being observed, as 

 the crystals are massed together very closely. 



On Plate YII. are shown six curves, three of which were plotted 

 from the calculated antilogs, and three others, which have broader Lines 

 were plotted from these antilog curves, and are reciprocal to them. 

 These latter three give some idea of the manner in which the intensity 

 of the light from the phosphorescing substances vary. It will be seen 

 that a specimen of strontium sulphide (c) has ceased to phosphoresce 



