6o6 THE POPULAR SCIENCE MONTHLY. 



was calculated to give, when combined, a mixtui'e of a definite percent- 

 age 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 0"01 per cent, was prepared. 



Observations were then made of the spectrum of each specimen, 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 Cd 10, 6, 1, 0'15 

 Lead + Zinc, " Zn 10, 5, 1, 0-1 



Lead -l- Magnesium, " Mg 10, 1, 0-1, O'Dl 



The observations showed that the lines of the smallest constituent 

 disappeared as the quantity got less. Although we had here the 

 germs of a quantitative spectrum analysis, the germs only were pres- 

 ent, because from the existence of several " critical points," and great 

 variations due to other causes, the results obtained were not constant. 



In a subsequent research on the gold-copper alloys used in the 

 coinage, Mr. Roberts, the Chemist of the Mint, and myself were able 

 to show that the shortening in the length of the lines by reduced quan- 

 tity was such a definite physical effect following upon reduced quantity, 

 that a difference of j^q^qq part of copper in gold could be detected. 



We are now in possession of the facts utilized in the work which 

 has led up to the subject discussed in the present paper. 



They have been utilized along two perfectly distinct lines of 

 thought : 



(1) They have been used in an attempt to enable us to produce a 

 spectrum of a substance free from lines due to the impurities which are 

 almost always present. 



(2) They have been used to indicate the existence and amount of 

 dissociation when acknowledged compounds have been submitted to the 

 action of different and increasing temperatures. 



I will deal with (1) first. 



The elimination of impurity lines is conducted as follows : The spec- 

 trum of the element is first confronted with the spectra of the sub- 

 stances most likely to be present to impurities. This is most conve- 

 niently done by photographing the spectra on the same plate one above 

 the other, so that common lines are continuous. 



The retention or rejection of lines coincident in two or more spectra 

 is detennined by observing in which spectrum the line is thickest ; 

 where several elements are mapped at once, all their spectra are con- 

 fronted on the same plate, as by this means the presence of one of the 

 substances as an impurity in the others can be at once detected. 



