ME, AY. CEOOKES ON THE ATOAIIC WEIGHT OP THALLIUM. 
303 
Thallium is likewise frequently present in copper and commercial salts of this metal. 
In Spain a very impure copper is prepared in the following way : — Copper pyrites is 
allowed to oxidize in the air, and the resulting sulphate of copper is washed out ; scrap 
iron is now placed in the liquid, which causes the copper to precipitate in the powdery 
state. The metal is then collected together, dried, strongly compressed, and heated to 
the melting-point. It is brought over to this country in the form of rectangular cakes, 
weighing about 20 lbs. each, and is called “ cement copper.” The sulphide of thallium, 
oxidizing- to sulphate along with the sulphide of copper, is washed out by the water, and 
precipitated with the copper by the iron. The two metals alloy together. 
Thallium is present in tolerable quantity in lepidolite from Moravia, and in mica from 
Zinnwald. It has likewise been found in the deliquescent “ sel a glace” from the 
mother-liquors of the salt-works at Nauheim. This consists of a mixture of the chlorides 
of magnesium, potassium, and sodium, with relatively considerable quantities of chlo- 
rides of rubidium and caesium, and sensible traces of chloride of thallium. Thallium is 
also met with in the mother liquors in the sulphate-of-zinc works at Gozlar, in the Harz. 
Noedenskjold has found in the copper-mine of Skrikerum, in Norway, a native sele- 
nide of copper, silver, and thallium, containing about 18 per cent, of thallium. It 
occurs in the form of lead-grey compact masses, having the hardness of copper glance and 
a spec. grav. of 6’9. This mineral has been named Crookesite by its discoverer. From 
the general association of selenium, copper, silver, and thallium in iron and copper 
pyrites it is probable that the thallium is here present in the form of Crookesite dissemi- 
nated through the mass. 
Th6 optical process of detecting thallium in a mineral is very simple. A few grains 
of the ore are crushed to a fine powder in an agate mortar, and a portion taken up on a 
moistened loop of platinum wire. Upon gradually introducing this into the outer edge 
of the flame of a Bunsen’s gas-burner, and examining the light by means of a spectro- 
scope, the characteristic green line will appear as a continuous glow, lasting from a few 
seconds to half a minute or more, according to the richness of the specimen. By 
employing an opaque screen in the eyepiece of the spectroscope to protect the eye from 
the glare of the sodium line, thallium may be detected in half a grain of mineral, when 
it is present only in the proportion of 1 to 500,000. The sensitiveness of this spectrum 
reaction is so great that no estimate can be arrived at respecting the probable amount 
of thallium present. 
Many samples of commercial sulphuric acid and yellow hydrochloric acid contain 
thallium. The source in these cases is evidently the pyrites used in the sulphuric-acid 
works. 
PBEPABATION OF COMMEBCIALLY PUBE THALLIUM. 
a. From the Flue-dust of Pyrites-burners . — This is by far the most economical source 
of thallium at present known. In burning thalliferous pyrites for the purpose of manu- 
facturing sulphuric acid, the thallium oxidizes along with the sulphur, and is driven off 
2 s 2 
