326 
MR. W. CROOKES ON REPULSION RESULTING EROM RADIATION. 
PART III. 
125. In my previous papers on this subject* the experiments described have had for 
their object the demonstration of the broad facts of repulsion resulting from radiation. 
In Part I., after satisfying myself that the action was not due to air-currents or elec- 
tricity, I went rapidly over bodies of the most diverse chemical and physical characters, 
organic and inorganic, metallic and non-metallic, dense and light, in spheres, disks, and 
thin plates, endeavouring to find, from their behaviour when free to move in a vacuum, 
what conditions were necessary to obtain the strongest movement under the influence 
of radiation, and what were unnecessary. I ascertained that chemical constitution had 
little or nothing to do with the action. I said (par. 75) “ the law appears to be that 
the force exerted is in proportion to the extent of surface exposed, rather than in pro- 
portion to the mass. Much surface and extreme lightness are the requisites in selecting 
materials for the beam, index, or gravitating mass ; and when the masses have the same 
specific gravity and extent of surface, their position in respect to the source of heat 
determines the extent of movement. Thus ' a cylinder of pith is more sensitive when 
arranged for the heat to act on its side than on its end.” I tried many experiments on 
the circumstances governing the position of the neutral point during exhaustion, and I 
proved that, within experimental limits, the nearer the vacuum approached perfection 
the stronger was the movement due to radiation. 
In Part II. I described many improved forms of apparatus by which the move- 
ments due to radiation could be studied in a more complete manner and numerical 
results be obtained ; the action of the various kinds of radiation, from the obscure heat- 
rays emitted by copper at 100° C. to the blue and ultra-violet rays of the spectrum, was 
examined, the interference caused by passing the rays through various screens was 
shown, and the phenomena of the neutral point were further discussed. Experiments 
were described which satisfied me that the hypothesis of the movements being due to 
evaporation and condensation at the surface would not account for all the facts of the 
case ; and ample proof was afforded that “ to get the greatest delicacy in these apparatus 
there is required large surface with a minimum of weight,” an apparatus for the quan- 
titative examination of this law being described. 
126. Nearly all the experiments described in Parts I. and II. were made with the 
* Philosophical Transactions, yol. clxiv. (for 1874) p. 501, and vol. clxy. (for 1875) p. 519. 
