128 THALLIUM. 



obtained, which as it cools solidifies to a white mass 

 of acid sulphate of thallium. This is dissolved in about 

 ten times its weight of hot water, filtered, and allowed 

 to crystallize. It may be purified by recrystallization, 

 and if the solution be decomposed by metallic zinc, 

 or by the voltaic battery, pure thallium is abundantly 

 and easily obtained. It may be melted in an iron 

 crucible heated over a gas flame, maintaining a current 

 of coal gas through the crucible to prevent oxidation. 



Thallium and its compounds are most easily and 

 certainly detected by spectral analysis. The spectrum 

 is characterized by a single bright green line coinci- 

 dent with Ba, . It is, however, usually perceptible 

 for but a moment, and its intensity and duration do 

 not safely indicate the amount of thallium present in 

 sulphides, flue-dust, &c. 



To find thallium in native sulphur the latter is 

 mostly dissolved by sulphide of carbon, and the residue 

 examined as above. In pyrites, flue dust, and sul- 

 phuric-acid chamber sediment, it may be usually de- 

 tected at once by the spectroscope. The sublimate 

 obtained by strongly heating finely pulverized sul- 

 phides in a closed glass tube often gives the reaction 

 when none can be obtained from the substance itself. 



III. Thallium is a heavy metal resembling lead in 

 its physical properties. Its specific gravity is 11.81 

 to 11.91. The freshly-cut surface of the metal has a 

 bluish-white lustre resembling zinc, which quickly tar- 

 nishes in the air, a thin film of oxide being formed. 

 It is soft, malleable, and may be pressed into wire, 

 though its tenacity is weak. It produces a streak on 

 paper like graphite. It melts if Cheated in oxygen, 

 and burns with an intense green flame. It combines 

 directly with chlorine, bromine, iodine, sulphur, and 

 phosphorus. It is very soluble in nitric and sulphuric 

 acids, but the action of hydrochloric is slow even when 



