ON OUR KNOWLEDGE OF SPECTKUM ANALYSIS. l43 



to by Professors Liveing and Dewar, but this may only be due to a 

 difference in temperature. 



No systematic attempt has hitherto been made to connect together 

 the different spectra of the same element ; relations which seem to hold 

 in one case seem again disproved in others, and no object therefore 

 seems to be gained by entering more closely into this matter at present. 



Report of the GomTnittee, consisting of Professors Odling, 

 Huntington, and Hartley, appointed to investigate by '■means 

 of Photography the Ultra-Violet Spark Spectra emitted by 

 Metallic Elements, and their combinations under varying con- 

 ditions. Drawn top by Professor W. N. Hartley (Secretary). 



The chief objects to be gained from a knowledge of the character of the 

 Spark Spectra of Metallic Elements, and of the combinations of the 

 elements, are : — 



(1) A means of I'eadily identifying the metals by photographs of their 

 line spectra. 



(2) A knowledge of the alterations producible in the spectra of 

 metallic salts by the presence of various non-metallic elements. 



(3) A knowledge of the alterations in spectra caused by the dilation 

 of metallic solutions. 



(4) A possible means of performing rapid quantitative determinations 

 of metallic substances by the aid of photography, and obtaining perma- 

 nent records of the results. 



All these objects have been more or less completely attained, but on 

 account of the extensive inquiry which has been opened up, it is pro- 

 posed on this occasion to present only a preliminary report. 



(1) In order to simplify spectroscopic work, the time of exposure 

 required to produce the most characteristic spectra under various con- 

 ditions, such as intensity of spark and conductivity, &c., of the electrodes 

 has been carefully ascertained. 



(2) A long series of experiments has been made with the object of 

 comparing the spectra of various compounds in solution with those of the 

 elements they contain. In the process of photographing the spectra of 

 solutions it is desirable to eliminate all foreign lines as far as possible, 

 hence the selection of suitable electrodes was a matter of the first con- 

 sideration. Electrodes of gold, platinum, iridium, and other metals were 

 nsed, and those of gold proved decidedly the best, as containing the 

 fewest lines, and the metal being a most excellent conductor of elec- 

 tricity. 



All these metals are, however, useless compared with electrodes of 

 graphite. The spectrum of graphite consists of eleven or twelve insigni- 

 ficant lines due to the carbon, and about sixty-six lines and bands due 

 to air. 



(3) In comparing the spectra of solutions of salts with those of 

 metallic electrodes, it was found that in almost all cases the lines of metals 

 were exactly reproduced from the solution, the graphic character being 

 retained except in regard to their continuity. Discontinuous but long 



