381 
“A MESSAGE FROM THE STARS.” 
capillary tube to a gas passing through it is proportional to 
the surface. (It is, of course, assumed that the ordinary con- 
ditions of capillary attraction are understood. The mechanical 
force which draws a fluid into a capillary tube acts upon any 
gas or fluid passing through the tube.) Hence, as was ob- 
served by Poiseuille, the resistance of the passage of a liquid 
through a capillary tube is nearly the fourth power of the 
diameter of the tube. The porous solids possess, no doubt, 
a similarly reduced penetrability as that possessed by a con- 
geries of capillary tubes. It must, however, be remembered, 
that the times of diffusion through tubes or pores have no rela- 
tion to the transpiration of the same gases. Gfraham has given 
with much clearness the generally received hypothesis,* which 
bears so strongly on the final result of this paper, that it is 
necessary to state it in as few words as possible. 
A gas is supposed to consist of solid and perfectly elastic 
spherical atoms, which move in all directions, and are animated 
with different degrees of velocity in different gases. Confined 
in a vessel, the moving particles are constantly impinging 
against its sides (should we not recognise the influence of the 
force on the surface of the confining vessel attracting the 
atoms?), and occasionally against each other. Owing to the 
perfect elasticity of the atoms, no loss of motion arises from 
such collisions. If the containing vessel be porous, then gas 
is projected through the open channels by the atomic motion, 
and escapes. The external air, or gas, is carried inward in the 
same manner, and takes the place of the gas which leaves the 
vessel. The molecular movement is accelerated by heat and 
retarded by cold; the tension of the gas being increased in the 
first instance and diminished in the second. This hypothesis 
assumes that when the same gas is present both within and 
without the vessel, and therefore in contact on both sides of the 
porous plate, the movement is sustained without abatement, 
molecules continuing to enter and leave in equal number, 
although nothing of the kind is indicated by change of volume. 
Is it not rather that the force residing on the surfaces of the 
pores of the septum compels this movement in and out ? f 
An interesting application of this knowledge has been made 
by Mr. George F. Ansell, for the purpose of detecting fire-damp 
in our coal mines. The disastrous consequences arising from 
the explosions which from time to time occur in working coal, 
* See “ Mathematical Physics,” by John Ilerapath (1847), and the 
Memoirs of Bernoulli, Joule, Clausius, and others. 
f u On the Law of the Diffusion of Gases ” (Transactions of the Boyal 
Society of Edinburgh, vol. xii. p. 222). u On the Motion of Gases” (Phil. 
Trans. 1846, p. 573). “ Capillary Transpiration of Gases ” (Phil. Trans. 
1846, p. 591 ; and 1849, p. 349). 
