316 
ME. AY. CEOOKES ON THE ATOMIC WEIGHT OF THALLIUM. 
the pernitrate of thallium, no deposit whatever is visible in the liquid ; but should any 
be seen, a fraction of a grain of oxalic acid must be added with the water in the next 
operation. 
As soon as the nitrate is in the form of a clear liquid the apparatus is allowed to cool *, 
and after being disconnected from the pump is weighed in the air-balance, no particular 
precautions being taken, however, and the air having free access to the interior of the 
apparatus. 
Water is again added, the nitrate of thallium is dissolved and allowed to crystallize 
out, and the operation of evaporating the water under diminished pressure in the air- 
bath is repeated exactly as already described. 
The dry nitrate is again fused at 394° F., and after the whole apparatus is heated to 
about 420° F. for a few minutes it is allowed to cool, and is again weighed. If there 
has been loss of weight, the operation must be repeated till the weights are constant. 
When this is the case the apparatus must be disconnected from the Bunsen water- 
pump and attached by its extremity g, Plate XVI. fig. 12, to the Sprengel mercury- 
pump. The air is now exhausted as perfectly as possible, and when quite vacuous the 
tube is sealed up at i (fig. 12) by the application of a small spirit-flame. Care must be 
taken in doing this to lose no particle of glass, as the end of the tube g which is drawn 
off, having been included in the first weighing, must be carefully preserved and weighed 
along with the apparatus in all the subsequent weighings. 
The apparatus is now of the form shown in fig. 13. It contains nothing but the pure 
nitrate of thallium produced from the action of nitric acid on the thallium at first intro- 
duced, and is entirely free from air. It is now, with the loose piece of tube g belonging 
to it, to be weighed in the vacuum-balance at two different atmospheric pressures, with 
all the precautions already adopted in the previous weighings. 
When the data for ascertaining the weight of the glass apparatus and the nitrate of 
thallium are correctly obtained, the weight of the glass apparatus by itself has to be 
taken. For this purpose a hole is perforated in the tube i, as before described, by means 
of a blowpipe-flame, and water being introduced the nitrate of thallium is dissolved out, 
and by repeated washings ultimately removed. The completion of the operation is 
ascertained by evaporating some of the washing water almost to dryness, and testing by 
means of the spectroscope. The apparatus is then dried, connected with the Sprengel 
pump, and after complete exhaustion it is sealed up at h, the same precautions being 
taken to preserve the piece of tube now removed as were adopted in the previous 
sealing up. 
The empty apparatus is now to be weighed at different atmospheric pressures in the 
vacuum-balance with all necessary precautions, the two loose pieces of glass tube g and 
* If a considerable bulk of fused nitrate of thallium is allowed to solidify in a thin bulb, the glass is almost 
certain to crack, owing to unequal contraction. Many of my operations were spoilt by this cause. By keeping 
the apparatus in motion during solidification, so as to allow the nitrate to line the greater part of the inner 
surface of the bulb, this source of danger is avoided. 
