274 Scientific Proceedings^ Royal Dublin Society. 



In quantitative spectra obtained by sparking solutions, the degree of 

 persistency of lines is indicated by means of Greek letters, thus — 



(7 lines seen with strong solutions. 

 ^ „ „ ,, 1 per cent, solutions. 

 ■^ „ „ „ •! per cent, solutions. 

 ■^P „ „ „ '01 per cent, solutions. 

 w „ „ „ '001 per cent, solutions. 



Ill representing tlie persistency of the lines of vacuum-tube spectra I have 

 adopted tliese symbols when the absolute quantities necessary to develop the 

 lines are known, and assign to them the following values : — 



tj) represents the presence of -0001 grammes. 



X „ ,. .. .' -00001 



^ „ „ „ „ -000001 „ 



a, „ „ „ „ -0000001 „ 



Thus all the lines of manganese, lead, copper, and lithium obtained in 

 No. I (Plate XXIV.) are lines. 



On the Continent ' « ' has been used to represent the ultiniate lines of an 

 element, but when the actual quantity of a substance present is unknown, 

 the lines might be graded in accordance with tlieir persistency, thus — 



Hi representing tlie ultimate line or lines, 



ui representing the next in order of persistency, 



«3 representing tlie next in order of persistency. 



This notation would tlieu approximately correspond witli the previous 

 notation, and can be used when the absolute quantities are not known ; and I 

 have adopted it wlien necessary in the present investigation, and the 

 persistencies are thus recorded in column 8 of the tables. 



Manganese. 



Tlie standard solution of manganous cliloride was prepared by dissolving 

 2'824 grammes of pure manganous carbonate in hydrochloric acid, and diluting 

 the solution to 1 litre with distilled water. 



When tlie first photograph (Plate XXIV., No. I.) was being taken, there 

 appeared in the tube for about four minutes a green luminescence tinged with 

 pink. On viewing this with the liand-spectrosoope a great number of lines were 

 visible. As may be seen, many of these lines appear in the photograph, thus 

 showing that this extremely small quantity ('0001 gramme) of the metal will 



