296 
MR. W. CROOKES ON THE ATOMIC WEIGHT OF THALLIUM. 
The reservoir A being filled with mercury, the tap I is turned off and the reservoir is 
raised to the top of the slide, where it is supported by the piece T. 
On opening the tap f the mercury rises in the tube fh, and falling through the 
chamber N, carries with it the air contained in the tube It and in the apparatus or bulbs 
attached to the tube A, as in the ordinary Sprengel pump. At N the tubing is enlarged 
in order that the mercury may not be forced up the tube E, as otherwise frequently 
happens if the apparatus or mercury gets soiled. This liability of the mercury to 
run up is further obviated by raising the tube K above the level of the upper reservoir A. 
J, J are iron brackets supporting the apparatus. S is a large inverted glass receiver 
to collect the small portions of mercury which are unavoidably and constantly being 
spilled ; it contains a little weak solution of carbonate of sodium. The part of the tubing 
(j f A N forms a barometric siphon arrangement, which effectually prevents air getting 
into the enlargement N from the reservoir A, when the mercury has completely run 
out: in this case no harm whatever is done to the operation, the vacuum is not injured, 
and the exhaustion proceeds immediately on retransferring the mercury from the reser- 
voir H to the reservoir A, and raising A again into its place. The apparatus, as thus 
arranged, is readily manageable with certainty of obtaining a barometric vacuum. 
The mercury fall-tube of a pump in constant use frequently wants cleaning. I find 
the most effectual means of doing this is to put oil of vitriol into the funnel stopper A, 
and then, by slightly loosening the glass stopper, allow a little of the strong acid to 
be carried down the tube with the mercury. With care this can be effected without 
interfering with the progress of exhaustion. The residual acid in the chamber N does 
good rather than harm. When sufficient sulphuric acid has run into the fall-tube, the 
funnel stopper can be perfectly closed by filling it with mercury. 
The preceding description is that of the apparatus with the most recent improve- 
ments. During the determination of the atomic weight of thallium a pump was 
employed similar in detail, with the exception that, instead of the movable reservoir 
and flexible rubber tubing, a glass funnel with tubing of glass was used. The mercury 
passing from the funnel was broken up in its fall and freed from adhering air-bubbles 
by the insertion of two silk-covered thin iron wires extending from the funnel to the 
base of the tube. As equally perfect results can be obtained by both methods, the 
details should be considered as improvements for the sake of convenience rather than 
for accuracy. — 29 November , 1872.] 
Section III.— THE CHEMICALS. 
In this section I shall detail the methods adopted in the preparation of thallium and 
the reagents in a chemically pure state. 
Water. 
Ordinary distilled water is mixed with a little crystallized permanganate of silver, and 
boiled for about half an hour. An excess of sulphuric acid is next added, and it is again 
