222 N. D. C. Hodges—Mean Free Path of a Molecule. 
The mineral may then be considered as a basic uranous uran- 
ate, in which the basic uranium is replaceable by lead (and 
iron). That the uranium exists in pitchblende as U,O,, has 
been assumed without proof; the only related crystallized 
mineral heretofore analyzed is the questionable uranoniobite of 
ot laa who gives 15°6 p. c.=PbO,Nb,0, and SiO,, and 2°7= 
loss 
The acid reaction shown in the tube is unexplained. It was 
first attributed to sulphur, but by careful examination none 
could be detected. 
ArT. XXIX.—On the Mean Free Path of a Molecule; by 
N. D. C. Hopes. 
THE free path of a molecule is dependent on the amount of 
obstruction it meets with, on the density of the medium. Meyer 
gives for the mean free path on page 308 of his Kinetische 
Theorie der Gase, Pe ONE” Here N is the number o 
molecules in the unit volume. 
consider the length of path in a medium of variable den- 
sity. At the surface of a liquid, if there is no sharp transition 
from the liquid to the gaseous state, we shall have a succession 
of less and less dense vapors from where there is liquid to the 
surrounding atmosphere. The layers V (fig. 1) are what I refer 
to. The depth of these vapors, is of course, much magnified. 
I propose to find the pressure upon the particle, p, when the 
surface of the liquid is plain and when it is spherical. Taking 
molecules moving with any definite velocity, they will reach ? 
and give it an impulse, when they are at a distance from p 
than their mean free path. Now, the particles from below come 
from denser layers than those from above. A greater number 
will come from below than from above; there will be a tend- 
ency to drive p upward. To find this tendency, we must find 
