480 THE POPULAR SCIENCE MONTHLY. 



burners to the leaden chambers. In these deposits, the minute propor- 

 tion of thallium contained originally in the pyrites becomes concen- 

 trated, so as to form in some instances as much as eight per cent, by 

 weight of the dust. Independently, moreover, of its occurrence in 

 iron pyrites, thallium, though never forming more than a minute con- 

 stituent of the different minerals and mineral waters in which it 

 occurs, is now known to be capable of extraction from a great num- 

 ber and variety of sources. But from no other source is it so advan- 

 tageously procurable as from the above-mentioned flue-deposit ; and so 

 early as the autumn of 1863, at the meeting of the British Association 

 in Newcastle, the then mayor, Mr. J. Lowthian Bell, exhibited several 

 pounds, and Mr. Crookes no less than a quarter of a hundred-weight of 

 thallium obtained from this comparatively prolific source. In one 

 respect, the discovery of thallium presented even a greater degree of 

 interest than attached to the discovery of caesium and rubidium. For 

 whereas these two elements were at once recognized as analogues of 

 the well-known metal potassium, thallium can hardly be said, even at 

 the present time, to be definitely and generally recognized by chemists 

 as the analogue of any particular metal, or as a member of any par- 

 ticular family of elements. With each of such differently characterized 

 elements as potassium, lead, aluminum, silver, and gold, it is associated 

 by certain marked points of resemblance ; while from each of them it 

 is distinguished by equally well-marked points of difference. Hence 

 the necessity for subjecting thallium and its salts to a thorough chemi- 

 cal examination, so as to accumulate a well-ascertained store of facts 

 with regard to it. And, thanks to the careful labors of many chemists, 

 more particularly of Mr. Crookes, in London, and of Messrs. Lamy and 

 Willm, in Paris, our knowledge of the properties of thallium and of its 

 salts may compare not unfavorably with our similar knowledge in re- 

 lation to even the longest known of the metallic elements. Still, it 

 was not until our knowledge of indium had culminated in the deter- 

 mination of its specific heat, only last year, that the position of thallium, 

 as an analogue of indium and a member of the aluminum family of 

 elements, became unmistakably evident. 



Indium was first recognized in 1863, by Drs. Reich and Richter, in 

 the zinc blende of Freiberg, in Saxony, and by reason of the very char- 

 acteristic spectrum afforded consisting of two bright-blue or indigo 

 bands ; the brightest of them somewhat more refrangible than the blue 

 line of strontium, and the other of them somewhat less refrangible than 

 the indigo line of potassium. Since its first discovery, indium has been 

 recognized in one or two varieties of wolfram, and as a not unfrequent 

 constituent of zinc-ores, and of the metal obtained therefrom, but always 

 in a very minute proportion. Indeed, indium would appear to be an 

 exceedingly rare element, far more rare than its immediate predeces- 

 sors in period of discovery. Its chief source is metallic zinc that of 



