364 M. C. Lea — Rbntgen Rays not present in Sunlight. 



4. The panel just described was replaced by one of stout 

 book board. With the sun shining on this book board directly 

 and not through glass, paper marked with a saturated solution 

 of barium platinocyanide exhibited no indications of fluores- 

 cence when placed behind the board. 



5. Three thicknesses of Bristol board were pasted together, 

 a circle was cut out, to one side of which barium platinocyanide 

 was applied. The circle was then placed in a pasteboard tube, 

 (an arrangement, I believe, proposed by Prof. Magie.) When 

 the sun was looked at through this tube the barium salt exhib- 

 ited fluorescence. But the interposition between the card and 

 the sun of very thin aluminium foil sufficed to cut off the fluo- 

 rescence. 



These concurrent results seem to indicate the absence of 

 X-rays from sunlight. 



Charles Henry* quotes an opinion of H. Poincare that all 

 bodies whose phosphorescence is sufficiently intense, emit in 

 addition to luminous rays the X-rays of Pontgen, whatever 

 may be the cause of their fluorescence. Henry quotes confirma- 

 tory experiments of his own made with zinc sulphide. 



It seemed worth while to ascertain if this principle is of 

 general application. A dilute solution of uranin was exposed 

 to sunlight, using a large surface of solution so as to get the 

 best effect. A short distance over the surface was placed a 

 sensitive film protected by aluminium foil T ^ of a millimeter 

 in thickness and with a lead star interposed. Two hours 

 exposure gav*e no result. The experiment was repeated with 

 acid solution of quinine, with which five hours exposure gave 

 no result. ' 



I have also examined the Welsbach light for X-rays. This 

 light is usually burned under a chimney which increases the 

 brightness but interposes glass between the source of light and 

 the sensitive film. Even without a chimney the light is bright. 

 The experiment was therefore made both ways. No X-rays 

 could be detected. Nothing capable of passing through alumi- 

 nium foil T X F of a millimeter in thickness by five hours exposure 

 to the uncovered flame. 



*0. R. cxxii, 312; Ohem. News, Feb. 28, 1896, p. 98. 



