314 
ME. W. CROOKES ON THE ATOMIC WEIGHT OF THALLIUM. 
tion ; but if tlie inner and external thermometers differ, I again rarefy by a few strokes 
of the pump, and repeat the observation after waiting for a few hours longer. 
Having obtained the accurate weight in a rarefied atmosphere, the next step is to 
weigh the apparatus in air of the ordinary density. Air is allowed slowly to enter the 
balance through the U-tubes at the side, and in a few hours, when the inner and outer 
temperatures are uniform, the weight is again taken. 
For the final adjustment of the weight the rider may be used. I, however, prefer, as 
being more accurate, to place a weight slightly in excess in the pan opposite to the 
apparatus to be weighed, and then, having sealed up the balance, to exhaust a little 
beyond the point of equilibrium of weight, and continue the operation exactly as in 
weighing in a rare atmosphere. By taking care that the air contained in the balance 
shall only be half an inch or so rarer than the external atmosphere, the data afforded by 
the two weighings will be sufficient to enable the true vacuum-weight of the apparatus 
to be calculated with accuracy. 
This method of ascertaining minute differences of weight, not by the addition to, or 
subtraction of, material weights from one arm of a balance, but by varying the density 
of the air in which the operation is performed, is, I believe, attended with a greater 
approach to accuracy than the method generally adopted. It can, however, only be 
adopted when the weights and the substance weighed differ in specific gravity. 
The data for ascertaining the weight of the apparatus and the thallium it contains 
have now been obtained. The next operation is to convert the thallium into nitrate. 
For this purpose, the tube g (Plate XVI. fig. 12) must be opened ; and to effect this 
without any risk of losing particles of glass, I gently warm the extremity in a spirit-lamp, 
and then apply the tip of a blowpipe-flame to the warm glass at g. The atmospheric 
pressure outside acting against the vacuum inside, immediately perforates a small hole 
through the glass, into which the air rushes. 
Some, nitric acid, purified in the manner described, is now removed from the bulb in 
which it has remained sealed up, and a little is introduced into the bulbs h and the 
globe b ; this is readily effected by alternately warming and cooling b , the perforation g 
dipping under the acid. Sufficient nitric acid must be introduced to three quarters fill 
the two lower bulbs, and also to moisten the thallium in the globe b. The apparatus 
is then placed in a horizontal position, and the quantity of acid in the bulbs is regulated 
so as to allow air-bubbles to pass in either direction and be washed without spirting 
acid out. 
No reaction, or scarcely any, between strong nitric acid and thallium takes place in 
the cold ; but on applying gentle heat the metal is attacked, and becomes rapidly con- 
verted into nitrate. The quantity of acid which is allowed to act at a time must be very 
limited ; and the temperature should not be higher than is sufficient to prevent the nitrate 
of thallium formed crystallizing on the metal and interfering too much with the action. 
As soon as the action ceases, a little nitric acid from the washing-bulbs is allowed to run 
into the globe, its place being supplied with fresh acid. When cold, the crystallization 
