ME. W. CEOOKES ON EEPULSION EESULTING- FEOM EADIATION. 
521 
December 11th, 1873, when illustrating my paper, I exhibited to the Society many of 
these new forms of apparatus. For the purposes of simple illustration, and for expe- 
riments where quantitative determinations are not required, I find a horizontal index 
suspended in a glass bulb the most convenient. The apparatus, with its mode of 
attachment to the pump, are shown in fig. 1. 
Fig. L 
a, b, c, d is originally a straight piece of soft lead-glass tubing 18 inches long, f- of 
an inch external and f internal diameter. At one end is blown a bulb, d e, about 3 
inches diameter. The part a b of the tube is drawn out to about half its original dia- 
meter, and bent at right angles. The tube is slightly contracted at c, and very much 
contracted and thickened at b. At a it is also contracted and cemented by fusion to 
a narrower piece of tube bent in the form of a spiral, and fitting by a mercury-joint 
into the sulphuric-acid chamber of the pump. The object of the spiral is to secure 
ample flexibility for the purpose of levelling the apparatus, and at the same time 
having a fused joint, f g is a very fine stem of glass, drawn from glass tubing, and 
having a small loop ( h ) in the middle. At each end of the stem is a ball or disk, made 
of pith, cork, ivory, metal, or other substance, li i is a fine silk fibre made from split 
cocoon-silk ; it is cemented by shellac at the upper end to a piece of glass rod a little 
smaller in diameter than the bore of the tube, and drawn out to a point, as shown. 
The contraction (c) in the tube is for the purpose of keeping this glass rod in its place ; 
when properly adjusted it is secured in its place by a small piece of hot shellac, care 
being taken not to cement the rod all round, and so cut otf the connexion between the 
air in the bulb and that in the upper part of the tube. The silk fibre is tied on to the 
loop of the glass stem at h. The length of the fibre is so adjusted that the stem and 
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