86 Prof. Draper on the Phosphorescence of Bodies. 



Before we can decide, there are c\'identl3' many preliminaiy 

 inquiries to be made. If chemical changes between the glowing 

 body and the air are disposed of, and the action is recognised to 

 be of a pm-ely physical or molecular kind, it is necessary to de- 

 temiine, — 1st, whether there is any expansion or contraction of 

 the shining body during its glow; 2nd, whether there is any 

 stractural change ; 3rd, whether there is any evolution of heat 

 along with the light ; or 4th, any development of electricity. 

 Each of these inquiries will now be taken up in succession. 



1st. Is there any change of volume in a phosphorescent body 

 during its glow ? 



I attempted to ascertain this by causing various bodies to 

 shine brightly when inclosed in glass vessels filled with water ; 

 so that if there was any expansion, the water might be pressed 

 out into a slender tube, and the amount of dilatation thereby 

 determined. The arrangement was as follows : — 



A glass tube, about two inches long and three-quai-ters of an 

 inch in diameter, was closed at its upper end by means of a plate 

 of polished quartz cemented air-tight. Immediately beneath the 

 quartz the phosphorescent body was supported. Through a 

 cork, which closed the other end of the tube, there passed a piece 

 of thermometer-tube bent on one side, and to it was affixed a 

 divided scale. The arrangement was supported on a suitable 

 stand, so that the quartz was uppei-most ; and at a little distance 

 above it the spark from a I.eyden jar could be passed between a 

 pair of stout iron wires maintained at an invariable distance, and 

 thus produce phosphorescence in the body. It may be remarked, 

 that these efi'ects of an electric spark do not take place well 

 through glass ; and hence a plate of quartz, which readily trans- 

 mits them, must be used. 



The large tube containing the phosphorescent body, must be 

 filled quite full of water free from air, as also niust be the ther- 

 mometer-tube to a given mark on its scale. If an electric spark 

 be now passed between the Avires to make the phosphorus shine, 

 it is clear that if there be any expansion or contraction of its 

 volume, there Mill be a corresponding movement in the water of 

 the thermometer-tube. 



On making the trial, and using in succession a crystal of violet- 

 coloured fluor-spar, a piece of flesh-coloured chlorophanc, and a 

 mass of Canton's phosphorus, the result in all cases was negative ; 

 for though these different substances glowed very brilliantly as 

 soon as the spark passed, there was not the smallest movement 

 perce])tible in the index liquid of the thermometer-tube. 



With a view of estimating the delicacy of the means here used 

 for detemiining any change in the volume of the spar, the solid 

 content of a piece of chlorophanc was determined by weighing 



