300 
MR. W. CROOKES OX THE ATOMIC WEIGHT OF THALLIUM. 
formic acid is obtained; above that temperature the barometer-gauge of the pump 
commences to sink; but the mercury descends very slowly until 330° is reached, when 
the decomposition of the oxalic acid into formic and carbonic acids becomes more rapid. 
Sulphuric Acid. 
After many attempts to prepare sulphuric acid free from arsenic by the distillation at 
a red heat of alkaline bisulphates, by dissolving sulphuric anhydride in water, and by 
decomposing sulphate of silver with sulphuretted hydrogen, I finally adopted Bloxam’s 
method of preparing it by means of sulphurous and nitrous acids (Journ. Chem. Soc. 
vol. xv. p. 52). 
The sulphurous acid is evolved from well-crystallized sulphite of sodium by the action 
of sulphuric acid, keeping the temperature as low as possible. The current of gas is first 
passed through a washing-bottle of water containing a little oxide of silver in suspension 
(which becomes converted into sulphite of silver, and then into a mixture of sulphate of 
silver and metallic silver), then through two U-tubes filled with small pieces of pumice- 
stone moistened with water. The pumice-stone should be previously purified from chlo- 
rides and fluorides by Stas’s method of igniting it twice with sulphuric acid. 
The nitrous acid is prepared by gently heating together nitrate of potassium, ferrous 
sulphate (both purified by repeated crystallization), and dilute sulphuric acid. 
The sulphurous acid and nitrous acid are conducted simultaneously by tubes into a 
large glass globe, a third tube serving for the introduction of steam. The three tubes 
pass into the globe through a glass plate in which three holes have been perforated. 
The glass plate and mouth of the glass globe are fitted to each other by grinding. No 
lute being used, sufficient air finds its way into the globe to keep up the reaction. By 
regulating the ingress of nitrous acid, of sulphurous acid, and of steam, the operation 
can be carried on continuously for many hours. 
The condensed liquid is next introduced into an apparatus blown from hard German 
glass, as shown in Plate NVI. fig. 6. a, b, and c are three bulbs about 3 inches diameter. 
The dilute sulphuric acid is introduced into the bulb a by means of the neck d , which 
is then sealed before the blowpipe at the contracted part. The end of the tube e is then 
connected with the Sprengel pump, the end f temporarily stopped up, and the whole is 
exhausted. The bulb b is immersed in a water-bath kept at the boiling-point, and a 
being gently heated the excess of water in the sulphuric acid goes off, and partly con- 
denses in the bulb c, which is kept cold, and partly becomes carried down through the 
pump by the falling mercury. 
Concentration of the acid proceeds rapidly ; and as soon as all excess of water has been 
thus eliminated, and the gauge of the pump shows that only aqueous vapour is present, 
the bulb c, containing water, is removed by applying a blowpipe-flame to the contracted 
portion of the tube g. Air is now admitted, the tube/* is connected with the Sprengel 
pump, and exhaustion again proceeded with. The bulb b is now kept cool, and the bulb 
a heated in a sand-bath. The atmospheric pressure must not be altogether removed, 
