1900.] on Bunsen. 445 



that besides Ka, Na, and Li, a fourth alkali metal must exist, and 

 all my time has been occupied in endeavouring to isolate some com- 

 pounds of the new substance. Where the presence of this body is 

 iudicated, it occurs in such minute quantity that I almost give up hope 

 of isolating it unless, indeed, I am fortunate enough tofiud a material 

 which contains it in larger amount." 



On November 6, I860, Buusen describes to me his further work on 

 the new metal as follows : — 



" I have been very fortunate with my new metal. I have got 

 50 grams of the nearly chemically pure chloro-platinic compound. 

 It is true that this 50 grams has been obtained from no less than 

 40 tons of the mineral water, from which 2*5 lbs. of lithium car- 

 bonate have been prepared by a simple process as a by-product. I 

 am calling the new metal " caesium," from " cassius," blue, on account of 

 the splendid blue line in its spectrum. Next Sunday I hope to find 

 time to make the first determination of the atomic weight." 



The rare combination of mental and manual dexterity charac- 

 teristic of Bunsen is nowhere more strikingly shown than in the 

 investigation of the caesiuni compounds. From these 17 grams of 

 caesium chloride, obtained as above described, he not only succeeded 

 in preparing and analysing all the more important compounds, but in 

 crystallising the salts in such a form that he was able to determine 

 their crystallographic constants, and then to supply all the necessary 

 data for fixing the position of this new element and its compounds in 

 relation to its well-known relatives potassium and sodium. 



All the world knows that, shortly alter his discovery of caesium, 

 the birth of another new alkali-metal, rubidium,* was announced by 

 Bunsen, and the application of spectrum analysis led to the isolation 

 of thallium in 1861, indium in 1863, germanium in 1866, gallium in 

 1875, and scandium in 1879, but alongside of these came announce- 

 ments of the discovery of other new metals whose existence was 

 more than doubtful. Concerning these he writes to myself : "The 

 frivolous way in which new metals are now discovered by dozens, and 

 sent forth into the world duly christened, is certainly no gain to science ; 

 only later inquirers will be able to decide what remains new and 

 serviceable out of this chaos of material." 



I had the good fortune to be associated with Bunsen for many 

 years in a somewhat difficult and most interesting research on the 

 measurement of the chemical action of light. So long ago as April 4, 

 1856, I brought before a Royal Institution audience the first fruits 

 of this work, and on May 22, 1863, and again, June 1, 1866, I 

 explained the further results which we had obtained. It is impossible 

 for me to do more on the present occasion than to remind you by an 

 experiment that it is the more refrangible rays of the solar spectrum 

 which have the greatest power of setting up chemical change, and to 



* Berlin Monatsh. 1861, vi. 273. 



