vi WORK AT MANCHESTER 135 



53, HARLEY STREET, LONDON, W. 



17 tk Jan., 1864. 

 MY DEAR SIR, 



I should like much to make a passing allusion in my Address 

 to the British Association to Bunsen's discovery of cesium 

 and rubidium when I am alluding to the mineral waters of 

 Bath. I shall only have room for a sentence on the subject, and 

 should be much obliged to you if you would have the kindness 

 to tell me in what year Bunsen discovered caesium, and in the 

 waters of what spring, and whether they were thermal waters, 

 as are those of Bath, and what were the principal mineral 

 ingredients in the spring. 



Would you give me the same information in regard to 

 rubidium ? 



If I mistake not, thallium is the only other metal brought 

 to light by spectrum analysis ? but not, I think, originally in 

 the waters of a mineral spring. But for this I may look over 

 Mr. Crookes' paper which I must have in the Phil. Trans., so 

 1 need not trouble you. 



I shall have no space for the history of spectrum analysis 

 as a method, but I should like if I can to put our friend 

 Bunsen's name in my address. 



Iron, I see, is the only metal of which a minute quantity 

 occurs in the Bath waters, just enough to be perceptible to the 

 taste, while they retain their heat, but I suppose I may 

 safely say of this or any thermal mineral water not yet 

 experimented upon, that we know not what contents may not 

 be revealed by spectrum analysis * when applied by such an 

 investigator as Bunsen. 



Believe me, 

 My dear Sir, 



Very truly yrs., 



CHAS. LYELL. 



p.S. Was not the proportion of caesium in the spring waters 

 very infinitesimal ? such as could never have been detected by 

 any other method ? Could I give any populaf expression to 

 the extreme minuteness of the quantity, such as a millionth 

 part of the proportions actually borne by some of those 

 ingredients that were previously recognised in mineral waters 

 as occurring in the most minute quantities ? 



May I not say that it was by solar light that Bunsen 

 discovered caesium and rubidium ? 



1 This prediction has been fulfilled by the discovery of radium, argon 

 and helium in the Bath waters. 



