82 



Prof. W. N. Hartley. 



[June 15, 



conditions, such as intensity of spark and conductivity, &c, of the 

 electrodes. This, in the instrument I prefer to use, is generally a 

 period of half a minute. ("Journal of the Chem. Soc," vol. xli, 

 p. 84, 1882.) 



(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 consideration, the method of working being almost entirely 

 dependent on this for its accuracy and value. No method like that 

 of Bunsen is convenient, in which charcoal points are employed in 

 conjunction with a spark from a coil without a condenser, by reason 

 of the prolonged exposure rendered necessary, the intensity of the 

 emitted rays being small. Electrodes of gold, platinum, iridium, and 

 other metals were used, and those of gold proved decidedly the best, 

 as containing the fewest lines and the metal being a most excellent 

 conductor of electricity. 



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

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

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

 due to air. 



The air-lines are easily recognised from their " physiognomie," as 

 M. Lecoq de Boisbaudran calls it, or as I have elsewhere described 

 this peculiarity in relation to spectrum photographs, their " graphic 

 character." In no case with the intensity of spark which I employ 

 and the normal exposure have I ever been troubled with the presence 

 of such impurities as may be contained in points of good Siberian or 

 Ceylon graphite. Such points have been submitted to the continuous 

 action of a condensed spark for something like ten hours at a time, 

 the same solution being used and the electrodes unaltered. It is 

 usual to take fresh electrodes for each solution. 



(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 lines, or in certain cases even short lines, appear as long lines 

 in the spectra taken from solutions. The peculiarities of the spectra 

 of magnesium, of cadmium, and iron, were exactly reproduced, line for 

 line, from the chlorides. An alteration was noticed in the spectrum 

 of graphite, the short lines became long, that is to say, discontinuous 

 became continuous lines, when the electrodes were wetted with water 

 or acids. 



An exceptional instance of variation in a spectrum was seen in that 

 of zinc. The pure metal exhibits a series of highly characteristic ex- 



