ME, W. CEOOKES ON THE ATOMIC WEIGHT OF THALLIUM. 
295 
to several thousandths of a grain in the space of two hours when a bulb of Bohe- 
mian glass three inches in diameter is exposed to a decided red heat in a gas-flame. 
Following the suggestion of Professor Stas, I have obviated this source of error by 
employing a bath of pure magnesia ; and find that the weight remains constant even at 
a nearly white heat. Baths of lime have sometimes been employed with similar satis- 
factory results. 
The special apparatus that I have used will be described in the processes in which 
they were required ; I need scarcely say that in no case were materials of untried purity 
employed. 
Improved Sprengel Vacuum-pump. 
[Before detailing the processes of the determination, it will be requisite to describe 
the means of producing a vacuum in the flasks and bulbs employed. In proceeding 
with the determinations, several additions and improvements have been made to the 
Sprengel pump as generally found in the laboratory. Working so much with this pump, 
I have endeavoured to avoid the inconveniences attending the usual mode of raising the 
mercury from the lower to the upper reservoir. 
The mercury is contained in a closed glass reservoir A (fig. 2, Plate XV.), perforated 
with a fine hole at the top. This reservoir is attached to a block, capable of free move- 
ment in a vertical line, and running in grooves, and connected with the lower reservoir 
by a flexible tube, g. 
When the whole of the mercury has run from the reservoir A, the reservoir and 
slide can be lowered by liberating the teeth of the cog-wheel K from the detent m ; at 
the same time a friction-brake is pressed against the cylinder. The aim of this arrange- 
ment is to permit the slide-block to fall steadily, swiftly, and without any injurious 
shock upon the block L. H is a glass reservoir, which receives the mercury after flowing- 
through the pump. When the reservoir A is emptied and has been lowered to the 
block L, the mercury from H is admitted into A by opening the tap I. f is another 
tap, to regulate the flow of mercury through the pump, whilst a third tap is at P. g is 
a flexible india-rubber tube, strengthened to withstand the pressure by being constructed 
of concentric layers of rubber and canvas, b is a tube filled with small glass beads and 
I containing concentrated sulphuric acid to absorb moisture * ; it is attached to l by 
means of a mercury joint, c , c, c are mercury joints, it being inconvenient to have the 
apparatus in one piece of glass tubing, e e is a barometer dipping into the same vessel 
! as the gauge-barometer P, the two thus forming a differential system, by which the rarity 
1 of the atmosphere in the apparatus to be exhausted can be easily estimated, d d is a 
! scale attached to the gauge. 
* Since writing the above description I have soldered a small glass tube to the lower part of b, turned up 
and terminating in a funnel-shaped mercury stopper. This enables me to draw off the old acid when weakened 
by absorption of moisture and to replace it by fresh acid, and also to pass different gases into any apparatus I 
may have under experiment. — W. C., July 1873. 
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