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XIII. 
OF ATMOSPHERES UPON PLANETS AND SATELLITES. By G. JOHNSTONE 
STONEY, M.A., D.Sc., F.R.S., F.R.A.S., Vice-President of the Physical Society, &e. 
[Received Ocroprr 25, 1897. | 
CONTENTS. 
PAGE PAGE 
Introduction, . ; . ; ; : . 805 | Chap. VIII. Of Venus, . j F : F . 819 
Chap. I. Of the Fundamental Facts, . : . 307 . IX. Of Mars, c ; : : : . 820 
A II. Interpretation by the Kinetic Theory, . 310 aA X. Of Jupiter, . ; : : j . 93822 
», III. Dynamical Equations, . : 2 tlh! aa XI. Of Saturn, Uranus, and Neptune, . - 3823 
i IV. Of the Earth, . - 5 : 3 a Bile) » XII. Of the Satellites and Minor Planets, . 3825 
os V. Extension of the Inquiry to other Bodies, 315 3, XIII. What becomes of escaped Molecules, 6 ile 
» VI. Ofthe Moon, . : 4 : 4 . 316 5, XIV. Former size of the Sun, 5 j . 226 
», VII. Of Mercury, . : : j : . 317 0 XY. Of MotionsinaGas, . : , . 827 
INTRODUCTION. 
THE present writer began early in the sixties to investigate the phenomena of 
atmospheres by the kinetic theory of gas, and in 1867 communicated to the Royal 
Society a memoir,* which pointed out the conditions which limit the height to 
which an atmosphere will extend, and in which it was inferred that the gases of 
which an atmosphere consists attain elevations depending on the masses of their 
molecules, the lighter constituents overlapping the others. This was disputed at 
the time, on account of its supposed conflict with Dalton’s law of the equal 
diffusion of gasest ; but physical astronomers now recognise its truth. 
On the 19th of December, 1870, the author delivered a discourse before the 
Royal Dublin Society, which was the first of the series of communications, of which 
an account is given in the following pages. One of the topics of that discourse 
was the absence of atmosphere from the Moon. This was accounted for by the 
kinetic theory of gas; inasmuch as the potential of gravitation on the Moon is 
such that a free molecule moving in any outward direction with a velocity of 
* See an extract from this Memoir on p. 307, below. 
+ According to the Kinetic Theory, Dalton’s Law will be true of mixtures of gases if the free 
paths of the molecules between their encounters are straight. This is the case, to an excessively 
close approximation, in all laboratory experiments; but the law ceases to hold at elevations in the 
atmosphere where the longer and more slowly pursued free paths are sensibly bent by gravity. 
TRANS. ROY. DUBL. SOC., N.S. VOL. VI., PART XIII. 3B 
