MR. W. CROOKES ON REPULSION RESULTING FROM RADIATION. 
355 
solar spectrum, on the action of light and heat on various surfaces other than black and 
white, and on some attempts I have made to measure the force of radiation. 
PART IY. 
186. In a former paper on this subject, communicated to the Royal Society March 20, 
1875, I gave some rough observations (110, 111) on the effect of the different rays of 
the electric and solar spectrum on the horizontal torsion-balance (102) ; and in a note 
(111) I said, “ Every thing is ready to try a series of experiments with the solar spectrum, 
as soon as sunshine is available. The results shall be communicated in a subsequent 
paper.” 
The apparatus which I have now used for this purpose is shown in figs. 12 & 13. 
Fig. 12 shows the horizontal torsion-balance; it is similar in appearance to the one 
described in par. 102. a b is a piece of thin glass tubing, sealed 
off at the end b, and ground perfectly flat at the end a. In the 
centre a circular hole (c) is blown, and another one (d) at the end, 
the centre hole being at the back, and the one at the end in front. 
The edges of these holes are ground quite flat, a, c, and d can 
therefore be sealed up by cementing flat transparent pieces of 
plate glass, quartz, rock-salt, &c. ( a , d, and d') on them. To the 
centre of ab an upright tube (e f) is sealed, having an arm 
(g) blown on it for the purpose of attaching the apparatus to 
the pump, h i is a glass index drawn from glass tubing, and as 
light as possible consistently with the needful strength. A long 
piece of this tube is first drawn out before the blowpipe, and it is then calibrated with 
mercury until a piece is found having the same bore throughout ; the necessary length 
is then cut from this portion, j Jc is a very fine glass fibre, cemented at j to a piece of 
glass rod, and terminating at Jc with a stirrup, cut from aluminium foil, in which the 
glass index ( Ji i ) rests. In front of the stirrup is a thin concave glass mirror, shown at #, 
silvered. The suspending thread is selected of the proper stiffness by the method given 
in par. 103. The small glass rod hung on to the end of the fibre to test its torsion 
weighs 15-46 grains ; its length is 90 millims., and its external diameter 3 millims. The 
selected fibre, having this glass rod suspended to it in air, was found to vibrate half 
oscillations in 35 seconds. 
The weight of the beam with the blackened pith ends is 0-891 grain ; the mirror and 
stirrup by themselves weigh 0‘87 grain ; therefore the whole beam, as suspended, weighs 
1-761 grain. The length of the beam from centre to centre of the pith squares is 147 5 
millims. This beam is so light, and the pith surfaces are so large, that its vibration in 
air, when suspended from the glass fibre, cannot be timed. Therefore to ascertain what 
the torsion of the fibre is with that weight suspended on it, I cemented a piece of pla- 
tinum weighing 1-543 grain (l-10th gramme) to the fibre, cut to the proper length,, 
Fig. 12. 
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