1069 
viz. the former inversely proportional to W/m, and the latter propor- 
. . . . m 1 . . 
tional to VWm,, so that D is proportional to | 7™. Similarly for 
m, ; 
(en $ Á : m, 
a trace of carbon-dioxide in hydrogen YD is proportional to Sat 
Mm, 
so that the ratio of the two coefficients is as mm, : m, = 44: 2. When 
the persistence is taken into account the ratio becomes quite different : 
the persistence is much smaller for the light hydrogen-molecules 
than for the heavy carbon-dioxide molecules, the consequence being, 
that the diffusion-coefficient is much less increased by the correction 
in the former than in the latter case. With the value given for the 
persistence in the papers mentioned the compensation was even 
complete, so that D for n,=0O became equal to D for n, =0, 
whereas for intermediate mixtures D obtained other values. 
Quite recently Miss A. SNEeTHLAGE, student of physics at Amsterdam, 
who is engaged on an investigation of various applications of the 
persistence-theory, has drawn my attention to the fact, that the 
expression given by me for the persistence cannot be correct. For, 
whereas for the case, that the masses m, and m, of the molecules 
are equal, it gives correctly the expression found by Juans, on the 
other hand for m, infinite it gives a negative value, although a 
simple calculation shows that its value must be nought in this case. 
In repeating the calculation it was found that on the former 
occasion an error had crept in which would have been noticed 
before, if the agreement with Jmans’s value for m, =m, had not 
erroneously been taken as a proof of its being correct. The present 
caleulation yielded the following expression for the mean persistence 
of a molecule m, colliding with a molecule m, : 
m, 1 m,” V(m, + m,) + Vm, 
NE Gm, +m) |E mgle(m, $m,” Vm, Hm) — Vn, 
which is identical with that found by miss Syeranace. The calcula- 
tion may be shortly reproduced; with a view to an easy com- 
parison with JrANs his notation and also, as far as possible, his 
method of calculation will be used. 
We take two molecules with definite velocities a and 6, which 
collide in all possible ways, and calculate for molecule a the mean 
velocity after the collision taken in the original direction of its 
motion. Maxwerr has proved, that this velocity is equal to that of 
the centre of mass projected on the same direction. Calling the 
angle between a and 4 &, this projection p is given by 
am, + bm, cos 3 
ee 7 
m, + ™, 
