542 
MR. W. CROOKES ON REPULSION RESULTING EROM RADIATION. 
An intimate connexion is thus shown to exist between the absorbing (and radiating) 
power of the surface on which radiation falls and the atmospheric tension at which the 
movement is reduced to a minimum. Further experiments on this subject are in 
progress. 
115. It has already been said that when radiation falls on a thin surface of pith, the 
neutral point is low, whilst with a moderately thick piece of platinum it is generally 
high. I have constructed a double torsion-apparatus by means of which these actions can 
be easily studied. Fig. 10 shows the arrangement of apparatus. It consists of a torsion- 
Eig. 10. 
apparatus constructed as the one shown in fig. 7 (102), with the exception of the arms 
being double. Similar parts in each drawing are shown by similar letters, 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 JI ) is blown, and two others are blown at the parts 
c and c'; the edges of these holes are also ground perfectly flat, a, c, c\ and c" can 
therefore be sealed up by cementing flat transparent plates of glass, quartz, rock-salt, 
&c., a , d, d 1 , d" on to them. At right angles to a b , and at the parts e, d, upright 
