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Rh principles of mathematics and molar physics for help, and these have told us something about the probable quantity of that atmosphere, though silent as to its possible quality. On this latter point, however, molecular physics turns out to have something to say; for an Irish gentleman, Dr. G. Johnstone Stoney, has recently made an ingenious deduction from the kinetic theory of gases bearing upon the atmospheric envelope which any planet can retain. His deduction is as acute as it appears from observation to be in keeping with the facts. It is this:—

The molecular theory of gases supposes them to be made up of myriads of molecules in incessant motion. What a molecule may be nobody knows; some scientists supposing it to be a vortex ring in miniature,—something like the swirl made by a teaspoon drawn through a cup of tea. But, whatever it be, the idea of it accounts very creditably for the facts. The motion of the molecules is almost inconceivably swift as they dart hither and thither throughout the space occupied by the gas, and their speed differs for different gases. From the observed relations of the volumes and weights of gases to the pressures to which they are subjected is deduced the fact of this speed and its amount. It appears that the molecules of oxygen travel, on the average, at the rate of