18.62.] 153 



These two metals are, however, seldom present with thallium in the 

 ores which I have examined. The nitric-acid solution is now to be 

 evaporated to dryness, the residue dissolved in hot dilute sulphuric 

 acid, and a piece of pure metallic zinc placed in the liquid; the 

 thallium will he at once precipitated in the form of a deep-brown 

 powder, which soon changes to a heavy black, granular precipitate. 

 The metal can be obtained in the coherent form by fusion in hy- 

 drogen. 



This method of analysis is given on the supposition that all the 

 above metals are present. It may generally be much abridged, as 

 the ore is seldom of so complicated a character. If there is a diffi- 

 culty in procuring perfectly pure zinc for the reduction of the sulphate 

 to the metallic state, this can be effected by passing a weak voltaic 

 current through the liquid, using platinum poles ; the metal will 

 then be precipitated in the reguline, or spongy state, according to the 

 strength of the current. I have not been very successful in reducing 

 the oxide by hydrogen. The current of gas carries the volatile oxide 

 away from the heated part of the tube before complete reduction 

 takes place. It is, however, probable, from an observation made 

 towards the conclusion of this experiment, that, with a longer tube 

 in proportion to the quantity of material, kept at a good heat through- 

 out its length, this plan might give good results, the metal being 

 considerably less volatile than the oxide. 



In many cases, when minute traces only of thallium accompany 

 large quantities of other metals, it may be advisable to repeat the 

 whole or some of the above operations, in order to purify this element 

 from foreign metals which may have escaped complete removal. 



I now pass on to a description of thallium and its chemical re- 

 actions. 



Thallium in the pure state is a heavy metal, bearing a remarkable 

 resemblance to lead in its physical properties. Its specific gravity is, 

 however, higher about 12. The freshly scraped surface has a bril- 

 liant metallic lustre not quite so blue in colour as lead, and it tarnishes 

 more rapidly than this latter metal. It is very soft, being readily 

 cut with a knife and indented with the nail ; it may also be hammered 

 out and drawn into wire, but has not much tenacity in this form. 

 It easily marks paper. The fusing-point is below redness, and with 

 care several pieces may be melted together and cast into one lump. 



