176 
POPULAR SCIENCE REVIEW. 
intromitted. The latter part is transmuted into rays of the 
kind which, by their dynamical action, produce heat of tem- 
perature as distinct from radiant heat This is what takes 
place on the bright side of each vane. On the blackened side 
none of the light is regularly reflected, and the whole is trans- 
muted into heat of temperature. The increase of temperature 
is therefore greater on the black than on the bright surface. 
The incident light thus produces an abnormal state of the atoms 
at and near the surface of the vane, analogous to the state of 
the superficial atoms of a body electrified by friction. He 
considers that the forces may result from the dynamical action 
of setherial undulations. There will be a decrement of setherial 
density from the dark towards the bright surface, and the atoms, 
being immersed in this variation of density, will be urged as if 
the vane were pushed on the black surface. 
Thinking, according to this theory, that the rotation of the 
vanes might be affected by the streams of a magnet in the 
neighbourhood, Professor Challis found on trial that such was 
the case, the rate of revolution being much reduced by the 
proximity of a magnet of considerable power. 
On June 15, 1877, Mr. Crookes again appeared before the 
Royal Society with a communication upon the 6< Influence of 
the Residual G-as.” He states that he has been assisted by 
Professor Stokes. He admits that the first and most obvious 
explanation was that the repulsive force was directly due to 
radiation. Further consideration, however, showed that the 
very best vacuum which could be produced might contain 
enough matter to offer considerable resistance to motion; in- 
deed, an instrument had been exhibited in 1876 to demonstrate 
this fact, which had also been proved by Dr. Schuster’s experi- 
ments. 
An apparatus had recently been constructed to discriminate 
experimentally between the viscosity of residual gas, friction of 
the needle point, and the torsion of the suspending glass fibre. 
It had been shown by this means that Professor Clerk Maxwell’s 
law of the independence of a gas’s viscosity and its density was 
practically correct. By using this apparatus the following 
measurements had been made : — 
1 . The logarithmic decrement of arc of oscillation when under 
no influence of radiation. 
2. The successive swings and final deflection when a candle 
shines on one end of the blackened bar. 
3. The appearance of an induction-spark between platinum 
wires. 
The first measures the viscosity, the second gives the force of 
the candle’s radiation, the third enables an estimate of the pro- 
gress of the vacuum to be made. 
