THALLIUM. 129 



hot, owing to the insolubility of the chloride. It forms 

 alloys with most of the metals. 



Thallium forms two oxides a protoxide and a 

 sesquioxide. Both the oxides dissolve readily in acids, 

 forming definite crystallizable salts, soluble in water ; 

 there are also a few insoluble salts obtained by double 

 decomposition. 



IV. Detection. 



1. In the dry way. The most characteristic property 

 of thallium is the intense green color which the metal 

 or any of its compounds communicates to a colorless 

 flame. This color examined in the spectroscope appears 

 as one intensely brilliant and sharp green line. The 

 spectral reaction is very delicate, the five-millionth part 

 of a grain of the sulphate being sufficient to produce 

 it. Thallium salts when ignited generally fuse below 

 redness, and then volatilize; some of them, however, 

 as the sulphate and phosphate, will stand a bright red 

 heat without change : the chlorides, on the other hand, 

 distil over with vapor of water. On charcoal before 

 the blowpipe they volatilize, giving an intense green 

 color to the flame. 



2. In solution. Salts of the protoxide are for the 

 most part colorless, unless the acid itself is colored. 

 They are mostly soluble in water, neutral to test paper, 

 and have a slight metallic taste. Their aqueous solu- 

 tion is rapidly precipitated in metallic crystals by zinc, 

 and slowly by iron. Hydrosulphuric acid added to a 

 solution of a protoxide salt containing a weak acid, 

 such as carbonic or acetic, separates the whole of the 

 metal in the form of a deep brown sulphide; from 

 solutions of the peroxide salts of the stronger acids, 

 such as the sulphate or nitrate, hydrosulphuric acid 

 precipitates nothing if the acid is in excess, and only 

 a small portion of the metal if the solution is neutral. 

 Sulphide of ammonium precipitates peroxide salt com- 

 pletely, the precipitated sulphide being insoluble in 



