X.] 



THE "SERIES" EVIDENCE. 



91 



Single lines. 



Doubles. 



Triplets. 



triplets are formed in exactly the same way. We can thus appreciate- 

 the enormous labour which has been faced by the inquirers I have 

 named in working out from the spectra of a great many substances 

 and from all the different regions of the spectrum, visible and photo- 

 graphic, these delicate triplets. In a great many cases they do not 

 represent the strongest lines, those most easily seen, and some want a. 

 great deal of looking for. 



These investigations show that in some cases the series have repro- 

 duced the same chemical group, but in some instances the series 

 groupings, so to speak, are quite different from the chemical group- 

 ings. 



The facts so far ascertained are as follows : 



Grroup 1 .. Lithium, sodium, potassium, rubidium, ca?sium. 



,,2 .. Copper, silver, (gold?). 



,, 3 .. Magnesium, calcium, strontium. 



4 . . Zinc, cadmium, mercury. 



5 .. Aluminium, indium, thallium. 



Iii the group of lithium, sodium, potassium, the series sequence- 

 follows absolutely the chemical sequence. But when we come to the 

 chemical group calcium, strontium, barium we find it replaced by a 

 group, magnesium, calcium, strontium, while barium is not used at all. 

 That is a very remarkable departure, and it shows that we have to con- 

 sider the various conditions which we observe in passing from group 

 to group. 



From group to group with increasing atomic weights the series 

 advance towards the violet. Thus, as the limit of a series is repre- 



