CHAP, xii.] SPECTRUM OF A MIXTURE. 151 



remained. Their number was large in the case of elements of 

 low atomic weight, and small in the case of elements of high 

 atomic weight. 



3. When we used metals mechanically mixed, of the smallest 

 constituent, only the longest lines remained. 



On this point I must enlarge somewhat by referring to a series 

 of experiments recorded in the Philosophical Transactions (1873). 1 

 A quantity of the larger constituent, generally from live to 

 ten grammes, was weighed out, the weighing being accurate to 

 the fraction of a milligramme ; and the requisite quantity of the 

 smaller constituent was calculated to give, when combined, a 

 mixture of a definite percentage composition by weight (this 

 being more easily obtainable than a percentage composition by 

 volume). 



The quantities generally chosen were 10, 5, 1, and O'l 

 per cent. 



In a few cases, with metals known to have very delicate 

 spectral reactions, a mixture of O'Ol per cent, was prepared. 



Observations were then made of the spectrum of each speci- 

 men, and the result was recorded in maps in the following 

 manner : First, the pure spectrum of the smallest constituent 

 was observed, and the lines laid down from Thalen's map. 



The series thus mapped was as follows : 



Tin + Cadmium percentages of Cadmium 10, 5, 1, 0'15 

 Lead + Zinc Zinc 10, 5, 1, O'l 



Lead + Magnesium Magnesium 10, 1, O'l, O'Ol 



The observations showed that the lines of the smallest 

 constituent disappeared as the quantity got less. These ob- 

 servations then rendered possible such a general statement as 

 this, for instance, that if we take, say, some iron, observe its 

 spectrum, and then mix some manganese with it, and observe 

 the spectrum of the mixture : if the quantity of manganese 

 1 Part ii. p. 479 et scq. 



