526 
ME. W. CEOOKES ON ATTRACTION AND 
38, 40, 41, 66, 70, 72); and observations more recently made, but only alluded to in 
par. 72, have proved that in air of ordinary density a cold body repels. 
On the supposition that air-currents are the motive power, the effects noticed when 
the source of heat is internal to the tube, and applied above the moving beam (37, 39, 
40, 41, 45), are inexplicable, whilst they are easily comprehended on the repulsion-by- 
radiation hypothesis. 
If an additional argument is necessary to show that air-currents are not the cause of 
the repellent action of a hot body, I bring forward the fact that the movement attains its 
maximum when there is no air at all present (54, 55, 68). 
79. Effects probably due to this repulsive action of radiation are constantly met with. 
I will instance the following : — 
Cohesion and adhesion are diminished by heat. This naturally follows if increased 
temperature augments the force of repulsion between the molecules. 
The phenomenon of the spheroidal state is probably due in some measure to a re- 
pulsive force exerted between closely approximated bodies, one of which is at a very 
high temperature. This action is generally supposed to take place only when one of 
the bodies is volatile, and the rapidly formed skin of vapour is held to be a sufficient 
cause of non-contact. I venture to anticipate that a condition similar to the spheroidal 
state will be found to obtain between non-volatile bodies. 
Many finely divided chemical precipitates, when incandescent in a platinum crucible, 
assume a remarkable mobility and flow about like water. Precipitated silica is an 
instance which will occur to chemists. A space can readily be distinguished between 
the powder and a hot capsule containing it. Electricity may, however, play some part 
in this action ; for precipitates, when heated, sometimes become sufficiently electrical by 
stirring with a glass rod to fly out of the basin containing them ; oxalate of lime pos- 
sesses this property in a remarkable degree*. 
80. It must, however, be remembered that my experiments show the action of hot 
bodies in air to be that of attraction, and that the repulsion by heat only becomes 
evident near upon a vacuum. It is seen, therefore, that radiant light or heat has an 
attractive or repulsive action, according to the medium in which it acts, corresponding 
results being furnished by cold. There appears to be an interfering action of air other 
than that of the currents caused in it by heat, which masks or overcomes the true action 
of heat ; but in a vacuum this interfering cause is absent, and radiant heat is free to exert 
its full repellent action, whilst cold or negative heat acts in the opposite direction. 
Heat and cold, heat present and heat absent — molecular activity and molecular rest — 
are therefore antagonistic in their action on a body free to move in empty space. The 
molecules of matter whose mode of motion constitutes heat are drawn together and 
* Fabaday’s ‘ Experimental Researches in Electricity,’ vol. ii. p. 163. 
