and Spectrum of Casium. 199 



The caesium spectrum, as we have procured it, is perhaps, 

 from the number, colour, and definition of its lines, the most 

 beautiful to be observed among all the alkali and earthy metals. 

 Kirchhoff and Bunsen, in the figure given by them (Philosophical 

 Magazine, vol. xxii. 1861), represent eleven lines. We find 

 without difficulty seven more lines, and observe further that some 

 of those figured by Kirchhoff and Bunsen are not mapped in 

 their correct positions. To enable other chemists to compare 

 their caesium preparations with ours, we will attempt to describe 

 the caesium spectrum as seen in our instrument, which has a 

 single flint-glass prism. 



Beginning at the left or red extremity of the spectrum, we 

 will indicate the lines in the order of their occurrence by Roman 

 numerals : I. is a red line of medium brightness nearly equi- 

 distant between the Fraunhofer lines a and B ; II. is a bright line 

 partly coincident with, but slightly to the left of and narrower 

 than the a line of lithium j III. is a faint line nearly approach- 

 ing Fraunhofer's line C ; IV. is the faintest of the red lines ; 



V. is a faint line midway between the a and /S lines of lithium; 



VI. is a bright red line midway between the sodium line and a 

 lithium ; VII. is an orange-red line of medium intensity directly 

 to the right of the a strontium line ; VIII. is a fine yellow line 

 just to the right of and close upon the sodium line. The posi- 

 tion of the green lines it is difficult to describe. First comes a 

 group of three, IX. X. and XL, which are separated by very 

 narrow spaces, and which are represented well in the spectrum 

 plate of Kirchhoff and Bunsen, though placed a trifle too far to 

 the right. Then, after an interval scarcely wider than the lines 

 themselves, come XII. and XIII. , which are very near each other. 

 After another space as broad as these lines we encounter XIV. 

 Midway between XIV. and XVI. is XV. The latter (XV.) coin- 

 cides with the dark line E. Finally, the two pale blue lines 



XVII. and XVIII. complete the list. 



For the convenience of those who may use spectroscopes of 

 the same construction as ours, we will mention the degrees on 

 the scale of our instrument which correspond to the caesium 

 lines. In our observations we have brought the degree 100 (10 

 on the scale) into the sodium line. Then the blue of strontium 

 is at 156°, the violet of potassium 257°, the red of potassium at 

 65-66°, the red of lithium at 80-81°. With this adjustment the 

 caesium lines are as follows, beginning with the red : I. 75°, 

 II. 80°, III. 82-3°, IV. 85°, V. 87-8°, VI. 91°, VII. 97-8°, 

 VIII. 101°, IX. 106°, X. 107-8°, XL 109°, XII. 111°, XIII. 

 112-13°, XIV. 114-15°, XV. 118°, XVI. 121°, XVII. 157-58°, 



XVIII. 160°. 



The position of the caesium lines on the scale figured at the 



