304 
ME. W. CROOKES OK THE ATOMIC WEIGHT OE THALLIUM. 
by the heat. If the passage leading from the burners to the leaden chambers is only a 
few feet long, the greater portion of the thallium escapes condensation, and is carried 
into the leaden chambers ; it there meets with aqueous vapours, sulphurous and sulphuric 
acids, and becomes converted into sulphate of the protoxide of thallium. This being 
readily soluble both in water and dilute sulphuric acid, and not being reduced by con- 
tact with the leaden sides, remains in solution and accompanies the sulphuric acid in its 
subsequent stages of concentration, &c. If, on the other hand, the passage connecting 
the burners and chambers is 10 or 15 (or more) feet in length, nearly the whole of the 
thallium is condensed, together with the multiplicity of other bodies which constitute 
“ flue-dust.” Accompanying the thallium have been found mercury, copper, lead, tin, 
arsenic, antimony, iron, zinc, cadmium, bismuth, lime, and selenium, together with 
ammonia, sulphuric, nitric, and hydrochloric acids. The amount of thallium in these 
flue-deposits is very various: in many specimens it is not present at all, and in very few 
it amounts to as much as J per cent., although in some as much as 8 per cent, of 
thallium has been found. 
The following is the best plan for extracting thallium from the dust : — The dust is 
first heated to very dull redness, so as to allow the excess of sulphuric acid to drive off 
any hydrochloric acid which may be present, and is then mixed in wooden tubs with an 
equal weight of boiling water, and well stirred ; after this the mixture is allowed to 
rest for twenty-four hours for the undissolved residue to deposit. The liquid is then 
siphoned off, and the residue is washed, and afterwards treated with a fresh quantity of 
boiling water. The collected liquors which have been siphoned off from the deposit 
are allowed to cool, precipitated by the addition of a considerable excess of strong 
hydrochloric acid, and the precipitate, consisting of very impure chloride of thallium, is 
allowed to subside. The chloride obtained in this way is then well washed on a calico 
filter, and afterwards squeezed dry. Three tons of flue-dust treated in this way yielded 
as much as 68 lbs. of this crude chloride. 
The next step consists in treating the crude chloride in a platinum dish with an equal 
weight of strong sulphuric acid, and afterwards heating the mixture to expel the whole 
of the hydrochloric acid. To make sure of this, the heat must be continued until the 
greater part of the excess of sulphuric acid is volatilized. After this the mass of bisul- 
phate of thallium is dissolved in about twenty times its weight of water, nearly neutra- 
lized with chalk, and then filtered. On the addition of hydrochloric acid to the filtrate, 
nearly pure chloride of thallium is thrown down; this is collected on a filter, well 
washed, and then dried. The crude protochloride of thallium obtained by either of the 
above methods is added by small portions at a time to half its weight of hot oil of vitriol 
in a porcelain or platinum dish, the mixture being constantly stirred and the heat con- 
tinued till the whole of the hydrochloric acid and the greater portion of the excess of 
sulphuric acid are driven off. The fused bisulphate is now to be dissolved in an excess 
of water, partially neutralized with carbonate of sodium, and an abundant stream of 
sulphuretted hydrogen passed through the solution. The precipitate, which may contain 
