354 



Prof. J. Dewar on 



[May 2, 



(wave lengths 5,510 and 5,730) are near to, but more refrangible 

 than, well-known emission lines of those elements. 



The absorptions produced by the mixtures of vapours plainly offer 

 a wide field for further observation. At present we have not succeeded 

 in observing those produced by mixtures, of vapours other than those 

 here recorded ; and it seems needful to use tubes of a less fusible 

 material than iron, which, notwithstanding the protection of fire- 

 clay, very quickly gives way at the temperatures we have used. 



With regard to those above described, we may observe that there 

 is in the solar spectrum an absorption line, hitherto unaccounted for, 

 closely corresponding to each of them. Thus, on Angstrom and 

 Thalen's map there are dark lines at 6,580 and 6,585, with more or 

 less continuous absorption between them, a broad dark line between 

 6,474 and 6,475, and a dark line at 5,300. There are also dark lines 

 nearly, if not exactly, coincident with the series of seven bright lines 

 of magnesium above described, which we have not seen strictly re- 

 versed. The coincidences of the series of the solar spectrum hitherto 

 observed have, for the most part, been with lines given by dense 

 electric sparks ; while it is not improbable that the conditions of tem- 

 perature, and the admixtures of vapours in the upper part of the solar 

 atmosphere, may resemble much more nearly those in our tubes. 



We intend to pursue our observations, using higher temperatures, 

 if we can obtain tubes which will stand under those circumstances. 



III. " Preliminary Note on Experiments in Electro Photometry." 

 By Professor James Dewar, F.R.S., Jacksonian Professor 

 University of Cambridge. Received March 26, 1878. 



Edmond Becquerel, in the year 1839, opened up a new field of 

 chemical research through the discovery that electric currents may be 

 developed during the production of chemical inter-actions excited by 

 solar agency. 



Hunt, in the year 1840, repeated, with many modifications, Bec- 

 querel's experiments, and confirmed his results. 



Grove, in 1858, examined the influence of light on the polarized 

 electrode, and concluded that the effect of light was simply an 

 augmentation of the chemical action taking place at the surface of the 

 electrodes. 



Becquerel, in his well-known work, " La Lumiere," published in 

 1868, gives details regarding the construction of an electro-chemical 

 actinometer formed by coating plates of silver with a thin film of the 

 sub- chloride, and subsequent heating for many hours to a temperature 

 of 150° C. 



Egeroff, in 1877, suggested the use of a double apparatus of 



