I9I0.] ON PHOSPHORESCENCE AND FLUORESCENCE. 271 



To determine more exactly the effects of temperature on the 

 duration of phosphorescence the following experiments, in which 

 Mr. J. F. Putnam assisted me, were made. Several sulphides of 

 known composition of the sort prepared by the method of Lenard 

 and Klatt, a number of which are now on the market, were cooled 

 by means of liquid air. The form of apparatus used is shown in 

 Fig. 2. 



The source of light was a flaming arc between carbons which 

 were filled with salts yielding an ultra-violet spectrum of great 

 intensity and unusual range. These carbons are known commer- 

 cially as the " brilliant white." They gave an arc which under 

 the conditions of our experiments excited the substances under ob- 

 servation to complete saturation in about six seconds. 



The lamp was of the vertical carbon type with a large upper 

 carbon, cored but not impregnated, and a smaller impregnated car- 

 bon below. The direction of the current was such as to make the 

 latter the positive terminal. Such a lamp burns several minutes 

 without feeding with an arc from one to two centimeters in length 

 and of sufficient steadiness for the purpose in question. 



A condensing lens of quartz 20 cm. focal length and 5 cm. 

 diameter threw an image of the arc upon the wide horizontal slit s, 

 the edges of which excluded light from both carbon tips. The 

 light after passing the slit was rendered parallel by the quartz lens /' 

 and fell upon a plane mirror m of speculum metal by which the 

 beam was reflected obliquely downwards at an angle of about 70° 

 through the similar lens /" which caused an image of the slit upon 

 the substance to be observed. The substance in a thin layer of 

 powder was contained in a shallow capsule at the top of a bronze 

 tube about 20 cm. long. When in position the top of this tube was 

 surrounded by a massive collar of copper which in turn was sup- 

 ported by . a tube of hard fiber which afforded excellent thermal 

 insulation. The whole was boxed in to prevent the gathering of 

 frost. Observations of the phosphorescent substance were made 

 through a horizontal tube inserted in the side of the box above the 

 capsule and having a rectangular prism p at the inner end, as shown 

 in Fig. 3. 



