ME. W. CROOKES ON THE ATOMIC WEIGHT OF THALLIUM. 
315 
of the nitrate of thallium entirely prevents the reaction between the metal and acid, but 
on warming, the salt dissolves and the action proceeds. The vapours of nitric and nitrous 
oxide, which are evolved in abundance, are washed by passing through the system of 
bulbs, and the reaction must be only just sufficient to cause them to pass through slowly. 
In course of time the whole of the thallium is dissolved, and the most tedious part of 
the process then commences — the evaporation of the excess of free acid. 
For this purpose an apparatus is used represented in Plate XVII. a is the apparatus 
connected by a wide tube, b, and a narrower glass tube with a bottle, c. This is in 
connexion with a Bunsen’s water-pump, d, having 18 feet fall of water, and capable 
of producing an exhaustion equal to 10 inches of mercury. The water is supplied to 
this pump by an independent pipe and tap attached to a large cistern, so that it can be 
allowed to work continuously day and night without interfering with the ordinary water- 
supply of the laboratory. The apparatus a is enclosed in a glass case, and stands in an 
air-bath, the temperature of which can be kept constant by means of a gas-regulator. 
The water of the pump being set in motion, and a temperature of about 250° F. being 
maintained in the air-bath, evaporation of the nitric acid commences, the vapour partly 
condensing in the bottle c , and partly being carried away through the pump. As the 
evaporation of the acid proceeds the temperature is gradually raised, until ultimately it 
becomes as high as 380° F., which must not be exceeded in this stage of the operation. 
In course of time (varying from a few days to as many weeks, according to the quantity 
of acid to be drawn off, and the size of the perforation through which it is to pass) the 
nitrate of thallium is left in the form of dry white crystals. 
The pump is then stopped, and air allowed to enter the apparatus by opening the 
pinch-cock, e, connected with the chloride-of-calcium tubes, f. The apparatus being 
cooled, water is added to the nitrate of thallium in the proportion of bulk for bulk, 
about 1 grain of oxalic acid* being dissolved in the water. Heat is then applied, and 
the solution boiled until all the nitrate of thallium is dissolved, forming a clear colour- 
less liquid, which deposits on cooling brilliantly white crystals of nitrate of thallium f. 
The nitric acid having been previously removed from the bottle c and the rest of the 
tubes, the apparatus is again fitted to the pump. It is heated in the air-bath, and the 
water gradually drawn out under diminished pressure, the temperature being kept a 
little below the point of ebullition of the liquid. When apparently dry the heat is very 
carefully raised to 394° F., at which temperature the crystals of nitrate of thallium melt ; 
a little froth at first breaks the surface, but this soon disappears, and the liquid becomes 
as clear and colourless as water. If sufficient oxalic acid has been added to decompose 
* If the action of nitric acid on thallinm is allowed to become too violent, or if the nitrate of thallium is long 
heated with excess of nitric acid, a little pernitrate of thallium is formed, which on subsequent fusion of the 
nitrate deposits a brown powder of peroxide of thallium. The oxalic acid is therefore added to decompose the 
pernitrate of thallium. The excess of oxalic acid disappears with the last traces of free nitric acid and water. 
t Nitrate of thallium is soluble in 9-4 times its weight of water at 60° E., and in less than one fourth of its 
bulk of boiling water. The crystals deposited on cooling are anhydrous. 
