84 



Prof. W. N. Hartley. 



[June 15.. 



alloy or a mineral, only those have recommended themselves to me 

 which depend upon the use of solutions ; and for the reason that 

 most alloys are not homogeneous, and the portion of a metallic 

 electrode exposed to the action of the spark is volatilised from one 

 point, and is too minute in quantity to represent the composition of 

 the mass. Now, the composition of a solution represents in every 

 part the composition of the entire mass dissolved ; it is, therefore, 

 quite unimportant how small a fraction of it is used for the purpose 

 of obtaining the spectrum of its constituents. 



It is a remarkable fact that at the present time we know little or 

 nothing of the sensitiveness of the spectrum reaction under various 

 conditions, notwithstanding that such knowledge is absolutely neces- 

 sary for the purpose of giving stability to numerous theories and 

 arguments which are based on spectrum observations. I have made 

 some experiments in this direction by determining the extent of 

 dilution which serves to modify in various ways the spectra of 

 solutions of metallic salts, and that which finally causes the extinction 

 of the most persistent line or lines. The sensitiveness of the reaction 

 varies with different elements and with the period of exposure, the 

 intensity of the spark, and other conditions; I have no difficulty 

 whatever, when working in the manner here indicated, in recognising 

 spectra yielded by solutions which contain no more than x^ooth of a 

 per cent, of calcium, silver, copper, and y^th of a per cent, of 

 manganese. It is necessary, however, for me to withhold a full 

 account of my experiments until I have determined the wave-lengths 

 of the lines in the various spectra under observation, for it is quite 

 impossible to describe the changes in the spectra without reference to 

 accurate measurements of the metallic lines. For some time past 

 Mr. W. E. Adeney has been working in conjunction with me at these 

 determinations, and I hope with as little delay as possible to have the 

 honour of submitting to the Royal Society all details here omitted, 

 both with regard to these new methods of analysis, and the wave- 

 length determinations. 



II. " On the Reversal of the Metallic Lines as seen in Over- 

 exposed Photographs of Spectra." By W. N. Hartley, 

 F.R.S.E., &c, Professor of Chemistry, Royal College of 

 Science, Dublin. Communicated by Prof. G. G. Stokes, 

 Sec. R.S. Received May 19, 1882. 



In preparing series of photographs of metallic elements when their 

 spectra are obtained by the action of a condensed spark passed between 

 metallic electrodes, I have been very careful to ascertain the exact 

 period of exposure of the sensitive plate to the rays, which will bring 



