Prof. R. Bunsen's Spectral- Analytical Researches. 531 



If the whole of the six members of this group be present 

 together, the recognition of lithium, thallium, and sodium is so 

 little influenced by the presence of the others, that quantities of 

 these elements which escape detection by any other means are 

 rendered evident with ease. 



The detection of traces of the other three elements when 

 they are mixed with these, as is usually the case in nature, is 

 not, however, so easy a matter, because in the presence of 

 potassium and sodium continuous spectra appear, which greatly 

 destroy the visibility of the other lines. 



The following process is adopted in order to remove these 

 continuous spectra — so far, at least, as to enable one to detect 

 with certainty the smallest traces of potassium, csesium, and 

 rubidium when present together, which can be recognized in no 

 other way. 



The chlorides are precipitated with platinum chloride as dou- 

 ble salts : the precipitation must take place in a cold concen- 

 trated solution, in order that the double salts may be thrown 

 down in a finely divided and, as far as possible, non-crystalline 

 condition. The precipitate is boiled in a platinum vessel, 20 

 or 30 times, with a little water, which is each time removed by 

 decantation, a small quantity being tested at intervals in the 

 spectrum-apparatus. For this purpose a few milligrammes are 

 brought on to a little piece of moistened filter-paper, which is 

 then surrounded by a very fine platinum wire and exposed to the 

 upper oxidation zone of the non-luminous flame until the paper 

 is completely burned away; the salt is then brought into the 

 fusion zone of the flame placed before the slit of the spectro- 

 scope. After the first treatment with boiling water, the potas- 

 sium-line generally appears beside those of lithium and sodium; 

 as the boiling continues the csesium and rubidium-lines become 

 gradually more and more apparent. Small traces of thallium, 

 which are often present in mineral waters, begin to make their 

 appearance only towards the end of the washing with boiling 

 water. 



2. Spectra of the Elements of the Alkaline-Earth Group, 



Nos. 7 to 10, Plates IV. and V., show the spectra of the 

 chlorides of calcium, strontium, barium, and magnesium. 



Sulphate of magnesium, purified from lime and iron by 

 repeated crystallization, served as the material from which mag- 

 nesium chloride was prepared. 



Barium and strontium chlorides, after repeated treatment 

 with hot alcohol, were purified by crystallization. 



Calcium chloride, prepared from limestone free from iron, 

 magnesia, aud manganese, was crystallized from absolute alcohol. 



2 M2 



