ME. W. CKOOIvES ON THE ATOMIC WEIGHT OE THALLIUM. 
299 
in water-baths, whilst the bulb c is immersed in a freezing-mixture. Distillation rapidly 
proceeds without ebullition, the acid almost entirely condensing into c. When about 
four fifths of the contents of b are distilled into c, the tube is sealed up at q before the 
blowpipe and the bulb b drawn off. 
The next operation is to clear the acid in c and the rest of the apparatus from nitrous 
vapours. For this purpose the mercury of the Sprengel pump is set gently in motion, 
and the apparatus is very moderately heated from time to time until the vacuum is as 
perfect as the tension of nitric-acid vapour will admit *. When this is effected, the 
bulb c is gently warmed in a water-bath, whilst d is immersed in a freezing- mixture. 
The temperature of c is so adjusted that distillation proceeds slowly without ebullition. 
When four fifths of the contents of c have come over, the mercury-pump working slowly 
all the time, the connecting-tube is sealed before the blowpipe at r, and c is drawn off. 
The flame being then applied at the contracted part s , the bulb d, containing the pure 
nitric acid, and having the appearance shown at Plate XVI. fig. 4 a, is removed. The 
acid may be kept unchanged for any length of time, provided it be not exposed to the 
light. When required for use, the end of one of the tubes is perforated with a blow- 
pipe-flame, as described above under the heading “Water.” By heating the bulb, any 
desired quantity of acid is driven out, when the remainder can again be sealed up. 
Oxalic Acid. 
Commercial purified oxalic acid is gently heated in a flat dish until the water of crys- 
tallization is removed. Bibulous paper is then placed over it, a paper cap over that, and 
the heat is increased until the oxalic acid sublimes. The sublimed crystals are removed 
from the inside of the paper cap, and introduced into the lower portion, «, of the glass 
apparatus shown in the diagram (Plate XVI. fig. 5). The tubes are contracted at b 
and c , and the end d is connected by means of an india-rubber connector with the Sprengel 
pump. The air is now completely exhausted from the apparatus, and it is immersed 
in a paraffin-bath to a little above the first contraction, b ; a thermometer is also im- 
mersed in the bath. 
The temperature is first raised to 200° F., and the exhaustion continued until all 
moisture disappears from the inside of the tube. The bath is then gradually raised to 
250°, and kept at that temperature till the oxalic acid has risen in vapour and condensed 
in the wide portion of the tube between b and c. The paraffin-bath is then taken away, 
and when the tube is cold it is removed from the pump by applying a blowpipe-flame 
to the contraction c ; this being repeated at b, leaves the sublimed oxalic acid perfectly 
pure in the bulb b c, and in a vacuum. 
In this apparatus oxalic acid commences to sublime below 200 D F. If the tempe- 
rature of the paraffin-bath be kept below 278° F. no permanent gas is evolved, and no 
* If acid vapours pass into the tubes of the Sprengel pump they do no harm, being carried down at once. 
The small quantity which condenses in the tubes may be afterwards removed by passing some distilled water 
through the pump, and then drying with warm air, or by passing oil of vitriol through the pump. 
