CusACK — Temperature — Phosphorescent Substances. 539 



when phosphorescing at different temperatures, recollecting that P is 

 less than unity so that log P is negative ; 



.•. -/ oc , Antilog n. 



When it is required to find the actual amount of light emitted by 

 a body it is necessary to find either the value of Iq or P, Iq being the 

 intensity of the light emitted by the lamp through the 1 cm. liole 

 when no paper is interposed. 



For an example let us suppose that at + 15° C. the light of the 

 photometer lamp had to be redueed by putting on seven papers until 

 the maximum brightness of the phosphorescing substance was equal 

 to it. By always starting work from normal, the lamp can easily be 

 got the same brightness by comparing it with the brightness of the 

 substance phosphorescing at 15° C. 



To find a measure of the relative brightness of a substance phos- 

 phorescing at + 15° and + 100°; if at + 100° 13 papers had to be 

 interposed in addition to the 7 at + 15° before the brightness was 

 reduced to the brightness of the phosphorescing body, it is necessary 

 to find the antilog of 7 and 20, and the reciprocals of these antilogs 

 will be a measure of the relative brightness of +15° and + 100° 

 respectively. These may of course be plotted to any convenient 

 scale. 



With regard to the manner in which the bodies under observation 

 were heated and cooled, I may observe that CO2 alone and in conjunc- 

 tion with sulphuric ether were the only cooling agents used ; with 

 these the temperatures of - 40° and - 80° were obtained. The tem- 

 peratures between normal and + 100° were obtained by immersing 

 the body contained in a nickel basin about one inch and a quarter 

 deep and an inch wide at the top, in heated water. A thermometer 

 dipped into the water, and when the brightness of the photometer 

 was, by trial, got equal to that of the body phosphorescing its maxi- 

 mum, the thermometer was read. A large volume of water was used 

 and the vessel was surrounded with several layers of felt, so the tem- 

 perature kept fairly constant ; but at all events the error arising from 

 misreading in temperature would only have been trifiing when com- 

 pared with those due to the photometer reading. 



Considerable difficulty arose in getting a suitable means of raising 

 the temperature above 100°. Mercury was at first tried, but the 

 vapour got over everything, and besides being injurious to breathe 

 was injurious to the instruments. Glycerine was also tried without 

 success, as also were several other organic compounds. Finally fusible 



