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 compared with those liberated in any chemical changes with which we are acquainted. Let us take an example: the atomic weight of chlorine is 35•5; this is not a whole number, it differs from the nearest by half a unit; it follows, therefore, that in the formation of 35•5 grammes of chlorine there must have been a change of mass of at least half a gramme. This involves the liberation or absorption of an amount of energy equal to that possessed by half a gramme moving with the velocity of light, i.e. 2•25 x 1020 ergs. This is about the amount of work required to keep the Mauretania going at full speed for a week, and must have been stored up or liberated from 35•5 grammes, or about an ounce of chlorine. We see that changes in the atom large enough to change the chemical character of the atom, i.e. to split an atom of one element up into different kinds of atoms, involve enormous transformations of energy; in fact the explosion of the atom in a few pounds of material might be sufficient to shatter a continent. We are living in the midst, nay, are made up of quiescent volcanoes; fortunately their slumbers are very sound.

Can we break up the atoms by physical means?

The amount of energy required to break up an atom has a very important bearing on the problem of splitting up the atom, in other words the transmutation of the elements by physical means. We know that the atoms of the radio-active elements break up spontaneously, and give rise to atoms of another kind. Thus radium emanation splits up into helium and radium A, and radium A again splits up. No one, however, has yet been able to influence the rate at which these transformations take place by any kind of physical treatment. Intense heat or pressure, and—what is much more remarkable—bombardment by the α rays given out by the radio-active