When the units of length and time are not restricted by the condition $c = 1$, the relation between the mass~$M$ and the energy~$E$ is \[ M = E/c^{2}. \Tag{(13.2)} \] Thus the energy corresponding to a gram is $9 · 10^{20}$ ergs. This does not affect the identity of mass and energy---that both are measures of the same world-condition. A world-condition can be examined by different kinds of experimental tests, and the units gram and erg are associated with different tests of the mass-energy condition. But when once the measure has been made it is of no consequence to us which of the experimental methods was chosen, and grams or ergs can be used indiscriminately as the unit of mass. In fact, measures made by energy-tests and by mass-tests are convertible like measures made with a yard-rule and a metre-rule. The principle of conservation of mass has thus become merged in the principle of conservation of energy. But there is another independent phenomenon which perhaps corresponds more nearly to the original idea of Lavoisier when he enunciated the law of conservation of matter. I refer to the permanence \PageSep{33} of \emph{invariant mass} attributed to our ideal billiard balls but not supposed to be a general property of matter. The conservation of~$m$ is an \index{Conservation!of matter}% \index{Matter!conservation of}% accidental property like rigidity; the conservation of~$M$ is an invariable law of nature. When radiant heat falls on a billiard ball so that its temperature rises, the increased energy of motion of the molecules causes an increase of mass~$M$. The invariant mass~$m$ also increases since it is equal to~$M$ for a body at rest. There is no violation of the conservation of~$M$, because the radiant heat has mass~$M$ which it transfers to the ball; but we shall show later that the electromagnetic waves have no invariant mass, and the addition to~$m$ is created out of nothing. Thus invariant mass is not conserved in general.