PROPERTIES OF MATTER IN THE GASEOUS STATE. 
835 
and since see - and per. p it appears that 
p-p x cTv 
So that with gas at a given density the smaller the surface the greater would be the 
intensity of the impulsive force ; and hence I was led to try the fibre of silk, with 
which I obtained evidence of the force at densities of half an atmosphere; whereas 
in the radiometer, with vanes something like 500 times as broad as the fibre of silk, 
the force does not manifest itself until the density is very small indeed. 
Earlier conclusions. 
119. The equations (124) and (140) show that both the forces and the consequent 
motion are, cceteris paribus, proportional to the heat communicated from the surface to 
the gas; for by equation (128) <x c —a'cc H where H is proportional to the heat commu¬ 
nicated from the surface to the gas. 
The necessity of such a relation was the subject of my former paper.* I then 
obtained the formula 
/=£ V4? 
To translate this into the symbols of the present paper 
and 
f=P,~p', 
d=gp 
- = Vi 
7T H 
18 c 2 ' 
According to my intention e should have been equal —, but from the 
which it was obtained it has the value given above (Appendix, note 5 (6)). 
have 
pc—p'= 
Vi H 
18 c 2 *' 
manner in 
Hence we 
The corresponding equation (Appendix, note 5 («)) derived from equation (140) is 
Pc—p = 
18 
s s 
11 x/' 
-fo/efer') I* Jl 
7T C 
s s 
C"a 
or when - is small 
s 
* Proc. Roy. Soc., 1874, p. 407. 
