282 Wells £ Foote — One Hundred Years of Chemistry. 



of this chapter, in articles published in the Journal (37, 

 67, 71, 1889). 



Applications of the Spectroscope. — The discovery in 

 certain mineral waters of the rare alkali-metals rubidium 

 and caesium by Bunsen and Kirchoff in 1861 was in conse- 

 quence of the application of spectroscopy by these same 

 scientists a short time previously to the identification of 

 elements imparting colors to the flame. Since that time 

 the employment of the spectroscope for chemical pur- 

 poses has been much extended, as it has been used in the 

 examination of light from electric sparks and arcs, as 

 well as from Geissler tube discharges and from colored 

 solutions. 



The metals rubidium and caesium are interesting in 

 being closely analogous to potassium and in standing at 

 the extreme electro-positive end of the series of known 

 metals. It should be noticed here that Johnson and 

 Allen of our Sheffield Laboratory, having obtained a 

 good supply of rubidium and caesium material from the 

 lepidolite of Hebron, Maine, made some important 

 researches upon these elements, accounts of which were' 

 published in the Journal (34, 367, 1862; 35, 94, 1863). 

 They established the atomic weight of caesium, thus cor- 

 recting Bunsen 's determination which was unsatisfac- 

 tory on account of the small quantity and impurity of his 

 material. Pollucite, a mineral rich in caesium, which had 

 been found in very small amount on the Island of Elba, 

 has more recently been obtained in large quantities — hun- 

 dreds of pounds — at Paris, Maine, and its vicinity. 

 This American pollucite was first analyzed and identi- 

 fied by the senior writer of this article (41, 213, 1891), 

 and later (43, 17, 1892 et seq.) the results of many inves- 

 tigations on caesium and rubidium compounds, in which 

 the junior writer played an important part, carried out 

 in Sheffield Laboratory, were published in the Journal. 



The application of the spectroscope led to the discov- 

 ery of thallium in 1861 by Crookes of England, and to 

 that of indium in 1863 by Reich and Richter in Germany. 

 Both of these metals are extremely rare, but they are of 

 considerable theoretical interest. Thallium is particu- 

 larly remarkable in showing resemblances in its different 

 compounds to several groups of metals. ] 



The spectroscope was employed again in connection 

 with the discovery of gallium in 1875 by Boisbaudran. 



