on the Existence of a Fourth Imponderable. Ill 



Taylor's Scientific Memoirs (vol. iii. part 12). He deter- 

 mines the place of the phosphorogenic rays in the spectrum, 

 after they have passed through a glass prism, and shows that 

 the fixed lines which occur in the chemical spectrum, occur 

 also among these phosphorogenic rays. 



In passing, I may mention that at the time I published my 

 account of these fixed lines (Phil. Mag., May 184-3), I had no 

 idea that any other chemist had seen them. It of course 

 soon appeared that M. Becquerel had some months previously 

 given an account of them to the French Academy. My result 

 was wholly independent and without any knowledge of his. 

 On comparing the two papers, it will be seen that thei'e is a 

 strong coincidence, not only in the manner of the experiment, 

 but even in the very phrases of description. It is this which 

 has drawn these passing remarks from me. Men who are 

 pursuing the same object, and using the same resources, will 

 employ even words that are alike, though they speak Ian* 

 guages that are different and live thousands of miles apart. 



On examining the plate given in M. Becquerel's paper, I 

 was struck with the close resemblance between the phospho- 

 rogenic spectrum and that tithonographic spectrum on iodide 

 of silver of which Sir J. Herschel has given an elaborate ac- 

 count (Phil. Mag., Feb. 1843). As far as the eye could judge 

 they seemed perfectly alike ; the tithonographic spectrum in 

 question was obtained by me in Virginia. This coincidence 

 was so striking that it appeared almost certain that the phos- 

 phorogenic emanations of Becquerel were the same as my ti- 

 thonic rays : there was the upper spectrum commencing at the 

 line G, and going beyond the furthest confines of the violet, 

 exerting a positive action ; there was also the lower spectrum, 

 commencing at the line F and going below the red ray, and 

 exerting a negative action; a phsenomenon absolutely the 

 same as that traced on the Daguerreotype plate. 



The phosphorogenic rays that come from the sun have the 

 same place in the spectrum, or are dispersed by the prism ex- 

 actly in the same way as the tithonic rays. To all appearance 

 these may be expressions for the same agent. 



We must I'emember, however, that the phosphorogenic 

 rays of the sun differ from those of an electric spark. Glass 

 to the former is transparent, to the latter it is not. 



Before, therefore, 1 can carry this argument to the point 

 on which 1 design it to bear, it is necessary to ascertain the 

 index of refraction of the rays of an electric spark to which 

 glass is impervious. 



This I proceeded to determine in the following way : — At 

 the distance of six inches from the terminations of two blunt 



