MR. W. CROOKES OK TIIE ATOMIC WEIGHT OF THALLIUM. 
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tin, arsenic, antimony, bismuth, lead, mercury, and silver, is separated by filtration, 
and the filtrate is boiled till all free hydrosulphuric acid is removed. The liquid is now 
to be rendered alkaline with ammonia and boiled ; the precipitate of iron and alumina, 
which generally appears in this place, is filtered off, and the clear solution evaporated 
to a small bulk. Sulphate of thallium will then separate out on cooling in the form of 
long, clear prismatic crystals. As sulphate of ammonium is much more soluble than 
sulphate of thallium, the latter can readily be separated from the small quantity of the 
former salt present. The two salts do not crystallize together. 
In order to avoid the inconvenience of driving off the excess of oil of vitriol in the 
decomposition of chloride of thallium, it is less troublesome, although not quite so 
economical, to proceed as follows: — Boil the chloride of thallium in solution of sulphide 
of ammonium for five minutes : decomposition takes place readily. Filter and wash 
with sulphuretted water till no more chlorine can be detected in the filtrate ; then 
dissolve the sulphide on the filter in dilute sulphuric acid, and treat the solution with 
ammonia &c., as above directed. 
In order to obtain the metal when working on small quantities of material, sulphate 
of thallium is dissolved in twenty times its weight of water ; the liquid is acidulated with 
sulphuric acid, and a current of electricity from two or three cells of Grove’s batteries 
is passed through it, platinum terminals being used. The appearance presented when a 
tolerably strong solution of thallium is undergoing reduction is very beautiful. If the 
energy of the current bears a proper proportion to the strength and acidity of the liquid, 
no hydrogen is evolved at the negative electrode, but the metal grows from it in large 
crystalline fern-like branches, spreading out into brilliant metallic plates, and darting- 
long needle-shaped crystals, sometimes upwards of an inch in length, towards the positive 
pole, the appearance being more beautiful than with any other metal. Some of the 
tabular crystals, as seen in the liquid, are beautifully sharp and well defined ; consider- 
able difficulty, however, is met with in disengaging them from the electrode, and removing 
them in a perfect state from the liquid. So long as thallium is present in the solution, 
no hydrogen is evolved with a moderate strength of current ; as soon as bubbles of gas 
are evolved, the reduction may be considered complete. The crystalline metallic sponge 
may now be squeezed into a mass round the platinum terminal, disconnected from the 
battery, quickly removed from the acid liquid, rinsed with a jet from a wash-bottle, and 
transferred to a basin of pure water. The metal is then carefully removed from the 
platinum, and kneaded with the fingers into as solid a lump as possible. It coheres 
readily by pressure, and will be found to retain its metallic lustre perfectly under water. 
When considerable quantities of thallium are to be reduced to the metallic state, it is 
convenient to employ metallic zinc for the purpose. In the course of twenty-four hours 
I have reduced upwards of a quarter of a hundredweight of the metal in the following- 
way : — Plates of pure zinc (which should leave no residue whatever when dissolved in 
sulphuric acid) are arranged vertically round the sides of a deep porcelain dish holding 
a gallon. Crystallized sulphate of thallium, in quantities of about seven pounds at a 
