310 
MR. W. CROOKES OK THE ATOMIC WEIGHT OE THALLIUM. 
so as just to fit a porcelain crucible; a hole is then turned in the centre of the lime, 
and a lump of lime cut into the form of a stopper. This apparatus is then raised to a 
temperature above the melting-point of thallium over a spirit-lamp, and the cavity in 
the lime is gradually filled with the metal, previously purified by one of the above pro- 
cesses, which is introduced in lumps. The stopper is then put on, and the heat raised 
to dull redness and kept so for half an hour ; after which the melted metal is poured 
into a lime mould, and preserved in a well-stoppered bottle under boiled water or very 
dilute acetic acid. 
The ingots of thallium purified by the various methods above described were kept 
separate, and were employed in the synthetical operations described further on. 
Section IY.— PROCESSES AND RESULTS. 
The processes and manipulation necessary to the determination of an atomic weight 
are at all times difficult and delicate, but especially so in the case of a metal such as 
thallium, so readily oxidizable. This strong tendency to combine with oxygen renders 
the ordinarily exact processes of weighing out pure metals inapplicable to the present 
purpose. The chances of contact with the oxygen of the atmosphere must be reduced 
to a minimum, and to this end the following modes of operation were devised. The 
method found to be the most accurate, and that adopted in repeating the determi- 
nations, will receive a description more detailed than the first and what may be termed 
approximative methods. 
Process of the Conversion of Thallium into Nitrate of Thallium. 
Thallium being a metal of very high atomic weight, the change in w 7 eight in the 
interconversion of its compounds is comparatively too small to be estimated with any 
approximation to accuracy. For instance, the conversion of acetate of thallium into 
chloride of thallium is an operation hardly to be effected without such loss as would 
seriously interfere with the calculated result*. The immediate conversion of the metal 
into one of its salts is therefore the method affording results less liable to be affected 
by errors in observation ; and the conversion of thallium into its nitrate has been that 
ultimately adopted. 
Pure thallium, obtained as described, is cut up into small bars with a very sharp steel 
knife, and dropped into a dish of pure water slightly warmed, and forming the sub- 
stratum to an atmosphere of carbonic acid in a vessel large enough to admit both hands 
easily. In this bath the original surface of the ingot is removed and rejected. The 
bars are then well rubbed with fine cambric to smooth down all sharp edges. Any 
pieces which contain pores are rejected f. 
A stoppered tube (Plate XVI. fig. 7) is half filled with water and weighed. The bars 
* An error of 0-05 grain in the weighing accumulates to an error of 095 in the atomic weight. 
t The upper surface of the fused lump is full of pores for a depth of one sixteenth of an inch ; one quarter 
inch is therefore removed for greater certainty. 
