1875.] The Mechanical Action of Light. 345 
Exhaustion is effected through a branch tube, h, projecting 
from the side of the upright tube. This is sealed by fusion 
to the spiral tube of the pump. The stopper de and the 
glass plates c and 0 are well fastened with a cement of resin 
and bees’-wax. 
The advantage of a glass-thread suspension is that the 
beam always comes back to its original position. 
An instrument of this sort, perfeCtly exhausted and then 
sealed off, is shown at work in Fig.5. It has pith plates at 
the extremities of the torsion beam. A ray of light from 
the lamp is thrown on to the central mirror, and thence re- 
flected on to the graduated scale. The approach of a finger 
to either extremity of the beam causes the luminous index 
to travel several inches, showing repulsion. A piece of ice 
brought near causes the spot of light to travel as much in 
the opposite direction. In order to ensure the luminous 
index coming accurately back to zero, extreme pre- 
cautions must be taken to keep all extraneous radiation 
from acting on the torsion-balance. The whole apparatus 
is closely packed round with a layer of cotton-wool about 
6 inches thick, and outside this is arranged a double row of 
Winchester quart bottles filled with water, spaces only being 
left for the radiation to fall on the balance and for the index 
ray of light to get to and from the mirror. 
However much the results may vary when the vacuum is 
imperfect, with an apparatus of this kind they always agree 
among themselves when the residual gas is reduced to the 
minimum possible; and it is of no consequence what this 
residual gas is. Thus, starting with the apparatus full of 
various vapours and gases, such as air, carbonic acid, water, 
iodine, hydrogen, ammonia, &c., there is not found at the 
highest rarefaction, any difference in the results which can be 
traced to the residual gas. A hydrogen-vacuum appears the 
same as a water- or an iodine-vacuum. 
The neutral point for a thin surface of pith being low, 
and that for a moderately thick piece of platinum being 
high, it follows that at a rarefaction intermediate between 
these two points pith will be repelled, and that platinum 
will be attracted by the same beam of radiation. This has 
been proved experimentally. An apparatus showing simul- 
taneous attraction and repulsion by the same ray of light is 
illustrated in Fig. 6. 
The pieces fg on the end of one beam consist of platinum- 
foil exposing a square centimetre of surface, whilst the 
extremities f’ g’ on the other beam consist of pith plates of 
the same size. A wide beam of radiation thrown in the 
