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 down possesses, is found in the explanation which has been given of it. That explanation is both so recent and so remarkable that I am glad here to have the opportunity of setting it forth, as it has an important application in the case of Mars.

Modern research has demonstrated that what we call a gas is in truth a mighty host of molecules far too small to be perceptible by the most powerful microscope. Each of these molecules is animated by a rapid movement, which is only pursued for a short distance in one direction before an encounter takes place with some other molecule, in consequence of which the directions and velocities of the individual molecules are continually changing. For each gas the molecules have, however, a certain average pace, which is appropriate to that gas for that temperature, and when two or more gases are blended, as in our atmosphere, then each molecule of the constituent gases continues to move with its own particular speed. Thus, in the case of the air, the molecules of oxygen as well as the molecules of nitrogen, are each animated by their characteristic velocity, and the same may be said of the molecules of carbonic acid or of any other gas which, in more or less abundance, may happen to be diffused through our air. For two of the chief gases the average velocities of the molecules are as follows: oxygen, a quarter of a mile per second; hydrogen, one mile per second; in each case the temperature is taken to be 64°C. below zero, being presumably that at the confines of the atmosphere. It will be noticed that there is a remarkable difference between the speeds of the two molecules here mentioned. That of hydrogen is by far the of any gas.