ME. W. CROOKES ON REPULSION RESULTING^ EEOM RADIATION. 
543 
tubes, f e, f' e', are sealed, one of them having an arm (g) blown into it for the purpose 
of attaching the apparatus to the pump, h, i, h', i' are glass beams made as light as 
possible consistent with the necessary stiffness, j Jc, j' Jc ' are glass fibres (103) cemented 
at j, j' to pieces of glass rod, and terminating at Jc, Jd with a stirrup cut from aluminium 
foil, in which the glass beams h, i, Ji', i' rest. In front of these stirrups are thin glass 
mirrors ( Jc , Jc'). At the ends of the beam ( Ji , i) are cemented very thin pieces of blackened 
pith, each 1 centim. square ; and at the ends of the other beam (Ji i’) are cemented pieces 
of platinum foil, also 1 centimetre square. At l and l' are narrow slits, with lamps 
behind them, so arranged that they send their rays of light respectively on to the mirrors 
{Jc, JJ), whence they are reflected back to the divided scale to. When the torsion-beams 
are riot acted on by any force, the rays of light both meet at zero (to), and there over- 
lap, the slightest movement of either beam causing them to separate. 
When the apparatus is full of air, a beam of radiation sufficiently wide to cover the 
whole window (c") being thrown upon the plates i, Ji’, the latter are instantly attracted, 
as shown by the movement of the reflected rays of light {Jc to , Jc' to). On exhausting 
the tube, and trying the effect of a hot body at the central window from time to time, 
it is seen that the movement of the pith surface {i) gradually diminishes, until at a 
certain point of exhaustion (in this apparatus at about 50 millims. below a vacuum) 
the neutral point for pith is obtained. On increasing the rarefaction the pith is 
repelled by radiation, the platinum continuing to be attracted. On exhausting the air 
still further (to about 28 millims.) the neutral point for the platinum surface is 
obtained, higher rarefactions producing repulsion of each when radiation falls on the 
pith and platinum surfaces {i, h'). 
At a rarefaction intermediate between the neutral point for pith (50 millims.) and 
the neutral point for platinum (28 millims.), the curious effect is produced of the same 
beam of radiation thrown into the window (<?") producing repulsion of the pith and 
attraction of the platinum, the two rays of light {Jc to , Jc' to) each moving to the right, 
and, if a piece of ice is presented to the central window, to the left. By adjusting the 
internal tension of the apparatus, a point may be reached (about 40 millims. below a 
vacuum) at which the repulsion of pith and the attraction of platinum are exactly 
equal, and then the two rays meeting at to do not separate, but together move to the 
right or left. 
116. A series of experiments have been tried with a view to ascertain what influence 
the state of surface of the substance submitted to radiation has on the amount or the 
direction of its movement. A torsion-apparatus was prepared similar to the one shown 
in fig. 7 (102), and having a thin disk of ivory at each end. One was coated with lamp- 
black, whilst the other retained its white polished surface. The average of a number 
of experiments showed that, under the influence of the same source of radiation acting 
for the same time (15 seconds), the white ivory was repelled so as to send the lumi- 
nous index 41 ‘5 divisions of the scale, whilst the blackened ivory caused, the index to 
