REPULSION RESULTING EROM RADIATION. 
509 
to exhaust temporarily, heating the straw by passing a spirit-flame along the tube, so as 
to diive off moisture. If, as is almost certain to be the case, one end becomes heavier 
than the other, equilibrium can 
Tig. 2. be restored, without much diffi- 
culty, by holding the spirit-flame 
for a few seconds under the 
heavier end, so as to slightly 
char the straw or other material. 
When in good adjustment and 
_ c a sufficiently sensitive the balance 
— ?— — - — ® 0 is ready for experiment. 
O 
25. The material with which 
I formed the masses at the ends 
includes platinum, brass, silver, lead, bismuth, aluminium, mag- 
nesium, glass, selenium, ivory, charcoal of different kinds, straw, 
cork, and pith. With each of these a large series of experiments 
were tried, and the experience gained with each was turned to 
account in making subsequent apparatus*. Certain differences, 
which I shall subsequently allude to, were noticed according to 
the material forming the gravitating mass ; but as soon as I suc- 
ceeded in obtaining the requisite degree of delicacy, the chief 
results were as decided as they were unexpected. 
26. The most delicate apparatus for general experiment is made 
with a straw beam having pith masses at the end. The general 
apparatus is shown in the annexed figure (fig. 2) : — 
A is the tube belonging to the Sprengel pump f. B is the 
desiccator, full of glass beads moistened with sulphuric acid. 
C is the tube containing the straw balance with pith ends. It is 
drawn out to a contracted neck at the end connected with the 
pump, so as to readily admit of being sealed off if desired at any 
stage of the exhaustion. D is the pump-gauge, and E is the 
barometer. 
27. The whole being fitted up as here shown, and the apparatus being full of air 
to begin with, I passed a spirit-flame across the lower part of the tube at b , observing 
the movement by a low-power micrometer ; the pith ball ( a b) descended slightly, and 
then immediately rose to considerably above its original position. It seemed as if the 
* It is only fair to acknowledge here the assistance which I have received during the progress of these 
experiments from my young friend and pupil, Mr. Charles H. Gijiingham. Without his skill with the blow- 
pipe and delicacy of manipulation with complicated apparatus, it would have been difficult for me to have 
carried out this investigation during the limited time I am able to devote to original research. 
f For a full description of this pump, with diagrams, see Phil. Trans. 1873, vol. clxiii. p. 295. 
MDCCCLXXIV. 3 Y 
