This verification of the general result~\Eq{(63.1)} for the case of a single particle gives another proof of the identity of gravitational mass with inertial mass. \index{Gravitational mass of sun!equality with inertial mass}% \index{Inertial mass!equal to gravitational mass}% \index{Mass!gravitational and inertial}% We see then that a particle is attended by a certain flux of the quantity~\Eq{(63.3)} across all surrounding surfaces. It is this flux which makes the presence of a massive particle known to us, and characterises it; in an observational sense the flux \emph{is} the particle. So long as the space is empty the flux is the same across all surrounding surfaces however distant, the radius~$r$ of the tube having disappeared in the result; so that in a sense the Newtonian law of the inverse square has a direct analogue in Einstein's theory. In general the flux is modified in passing through a region containing other particles or continuous matter, since the first term on the right of~\Eq{(60.3)} no longer vanishes. This may be ascribed analytically to the non-linearity of the field equations, or physically to the fact that the outflowing influence can scarcely exert its action on other matter without being modified in the process. In our verification for the single particle the flux due to~$\delta m$ was independent of the value of~$m$ originally present; but this is an exceptional case due to symmetrical conditions which cause the integral of $T^{\mu\nu}\, \delta g_{\mu\nu}$ to vanish although $T^{\mu\nu}$~is not zero. Usually the flux due to~$\delta m$ will be modified if other matter is initially present. For an isolated particle $m\, ds$~in any region is stationary for variations of its track, this condition being equivalent to~\Eq{(56.6)}. Hence for this kind of variation the action $8\pi \sum m\, ds$ in a region is stationary. The question arises how this is to be reconciled with our previous result (\SecRef{60}) that the principle of stationary action is untrue for regions containing matter. The reason is this:---when we give arbitrary variations to the~$g_{\mu\nu}$, the matter in the tube will in general cease to be describable as a \emph{particle}, because it has lost the symmetry of its field\footnotemark.\footnotetext {It will be remembered that in deriving~\Eq{(56.6)} we had to assume the symmetry of the particle.} The action therefore is only stationary for a special kind of variation of~$g_{\mu\nu}$ in the neighbourhood of each particle which deforms the track without destroying the symmetry of the particle; it is not stationary for unlimited variations of the~$g_{\mu\nu}$. \PageSep{146}