Prof. Bunsen on C&sium. 245 



do not exhibit this property ; and for this reason they have failed 

 to remark the hygroscopic nature of the chloride of caesium. 



In our memoir on the new metals* we have distinctly stated 

 that we have represented in our drawing only those lines which, 

 on account of their position, sharpness, or intensity, serve as the 

 best means of detecting the presence of the substances in ques- 

 tion. Messrs. Johnson and Allen remark with regard to this, 

 " Kirchhoff and Bunsen, in the figure given by them (Phil. Mag. 

 vol. xxii. 1861), represent eleven lines. We find without diffi- 

 culty seven more lines, and observe further that some of those 

 figured by Kirchhoff and Bunsen are not mapped in their correct 

 positions/'' In reply to this remark, we must regret that Messrs. 

 Johnson and Allen appear to have neglected to make an exact 

 comparison of their spectrum with ours, otherwise they would 

 have easily convinced themselves that the lines which they have 

 drawn correspond as closely as can be expected with those 

 which we represented, whilst they would at the same time have 

 noticed that three lines occur on our drawing which are not seen 

 at all in the caesium spectrum. These three lines were printed 

 in a large number of the lithographs by a mistake of the printer, 

 and lie close to the right of the lines marked VI., X., and XV. 

 by Messrs. Johnson and Allen. Setting aside these three lines, 

 introduced by a mistake of the lithographer, we have the follow- 

 ing agreement of the several readings : — 



1. 2. 3. 4. 



1 75 25 



II 80 30 



III 82-5 32-5 



IV 85 35 



V 87-5 37-5 



VI 91 41 41-7 



VII 97-5 475 47-6 



VIII 101 51 



IX 106 56 57-1 



X 107-5 57-5 58-1 



XI 109 59 



XII Ill 61 



XIII 1125 62-5 64-1 



XIV. .... 114-5 64-5 65-8 



XV 118 68 68-7 



XVI. .... 121 71 



XVII 157-5 107-5 106-3/3 



XVIII 160 110 109-0 a 



The first column contains the numbers by which Messrs. 

 Johnson and Allen have designated the lines observed by them in 

 * Phil. Mag. vol. xxii. p. 508. 



