158 [June 19, 



therefore assume that thallium is not present to any great extent in 

 the sun. Under the highest telescopic power of my apparatus, the line 

 appears to be absolutely identical in refrangibility with a sharp well- 

 defined line in the barium spectrum, to which Professors Bunsen 

 and Kirchhoff have given the name BaS. Want of material has 

 hitherto prevented me from taking accurate measurements of the 

 distance between the thallium-line and the principal lines of the solar 

 spectrum. 



This green line is an exquisitely delicate test for the presence of 

 thallium, and shows it to be a somewhat widely distributed element. 

 Many specimens of crude sulphur contain it (especially when rather 

 dark-looking). In most cases it is only necessary to set fire to as 

 large a piece of sulphur (less than a pea) as the platinum loop will 

 hold, and when it has nearly burned away to blow it out, and 

 then introduce it at leisure into the flame of the spectroscope, for 

 the thallium to show its presence by a bright-green line which will 

 flash for an instant into the field of view. Although the greater 

 part of the thallium is left behind after burning off the excess of 

 sulphur in this manner, some of it volatilizes, and consequently, if 

 the specimen gives no indications of thallium by this treatment, it 

 will be advisable to dissolve out as much of the sulphur as possible 

 with bisulphide of carbon, and then to test the residue in the 

 flame. 



Thallium is a constituent of very many mineral ores. Upon 

 examining a large collection of cupriferous pyrites from different 

 parts of the world, I found it present in more than one-eighth. It 

 is not confined to any particular locality ; neither does it seem to 

 bear any relation to the presence or absence of arsenic in the mineral. 

 I have, however, very rarely met with it in pyrites in which copper 

 was absent. In most cases it is only necessary to powder a small 

 fragment of the mineral and ignite a little of it in the flame on 

 a moistened platinum wire, for the green line to be distinctly 

 visible. 



If a thalliferous pyrites is finely powdered and then heated to 

 redness in a glass tube, as much as possible out of contact with air, 

 the sulphide of thallium, together with some free sulphur, sublimes 

 from it and may be condensed by appropriate arrangement. This 

 sublimate gives the thallium-line with great brilliancy. 



