MR. W. CROOKES ON REPULSION RESULTING FROM RADIATION. 
293 
ing the plates, or of making cones of them, is to produce a more favourable presentation 
to the inner surface of the glass bulb. Radiation falls from the candle on the 
aluminium ; some is reflected and is lost, but a portion is absorbed to be converted 
into thermometric heat or heat of temperature (171, 195, 278). Aluminium being a 
good conductor of heat, and the thickness of metal being insignificant, it becomes 
equally warm throughout, and a layer of molecular disturbance is formed on each 
surface of the metal. At a low exhaustion the thickness of this layer is not sufficient 
to reach from the metal cone to the side of the glass bulb ; as the exhaustion 
increases, this layer extends further from the generating surface, until at a sufficiently 
high exhaustion the space between the side of the glass bulb and the adjacent 
portion of the metallic cone is bridged oyer, and pressure is exerted between the two 
surfaces. A reference to the diagram (fig. 21) shows how this pressure will act. 
The direction "of pressure is indicated by dotted lines issuing from the metal cone ; 
it is assumed that the exhaustion is such as to allo\y the layer of molecular dis- 
turbance under the influence of the candle to extend fo;r 5 millims, from the surface 
of the metal, and I have considered that the pressure acts in a direction normal to the 
surface (really it will act in all directions, but for the sake of illustration I confine 
myself to the direction in which it acts with greatest force). The rays from the candle 
fall on the concave surface of the cone ; the substance being a good conductor of heat, 
molecular pressure is induced in the direction and to the distance shown by the 
dotted lines (fig. 21), the pressure being greater the nearer the layer is to the metal. 
Fig, 21. 
The pressure from the inside of the cone, and from the outside away from the side of 
the glass, is dissipated without acting, but the pressure between the glass bulb and the 
side of the cone nearest to it is active ; the cones, therefore, are pressed round in the 
direction of the arrows, and the motion has the appearance of attraction. 
313. When cones of clear mica are used, very little radiation is arrested by them 
(276, 2 77). Mica being almost perfectly transparent both to light and radiant heat, 
enough radiation is not absorbed to raise the temperature to the point required to pro- 
duce molecular pressure on the surface of the mica. The cause of the set of the two 
cones equi-distant from the candle is that the portion of the bulb nearest the candle 
gets warm, and this warm piece of glass does not act by radiation but is itself the 
repelling surface, through the intervention of the molecules rebounding from it with 
a greater velocity than that with which they strike it (219). 
