168 
POPULAR SCIENCE REVIEW. 
has occurred to me that, according to the Kinetic theory , a 
somewhat similar effect to that of evaporation must result 
whenever heat is communicated from a hot surface to gas” 
The particles which impinge on the surface will rebound with 
a greater velocity than that with which they approached, and 
consequently the effect of the blow must be greater than it 
would have been had the surface been of the same temperature 
as the gas. And in the same way, whenever heat is communi- 
cated from a gas to a surface, the force on the surface will be 
less than it otherwise would be, for the particles will rebound at 
a less velocity than that at which they approach. 
Of this important generalization of the view with which the 
paper started, mathematical demonstration is given. Convec- 
tion currents are said not to have much to do with Mr. Crookes’s 
experiments, as one can hardly conceive that much heat could 
be communicated to the gas or vapour in such a perfect vacuum 
as that he obtained, unless indeed the rate of diffusion varies 
inversely as the density of a gas. To this a note is added* 
which appears to go even deeper to the root of the matter 
than the text. “ Professor Maxwell,” it says, u has shown that 
the diffusion both of heat and of the gas varies inversely as 
the density ; therefore, excepting for convection currents, the 
amount of heat communicated from a surface to a gas would be 
independent of the density, and hence the force (f) (previously 
given as the force arising from evaporation) would be indepen- 
dent of the density ; that is to say, this force would remain 
constant as the vacuum improved, while the convection currents 
and counteracting forces would gradually diminish.” 
It will be seen that this valuable paper, which has been ab- 
stracted in considerable detail, ends by suggesting a far more 
general solution of the problem than that which is implied by 
its title. 
In November of this year occur the remarks of the President 
of the Royal Society already cited. 
What may be termed the second stage in the history of ra- 
diometry commences with a paper of Mr. Crookes’s, received by 
the Royal Society on March 20, 1875, and read on April 22* 
It is marked by the same high level of experimental power, 
the same unwearied patience in varying experimental condi- 
tions, and by the same combination of scientific imagination 
with accuracy of statement and mechanical aptitude as was the 
first. 
It describes necessary improvements in the Sprengel pump* 
and different forms of radiometric apparatus, by means of 
which the sensibility of the instrument is materially increased^ 
The bulb is surrounded with adiathermanous media, and with a 
second vacuum. Measurement is substituted for mere demon- 
