Both terms of our equation of motion are then small of the first order. If we neglect terms which, relatively to these, are small of the first order, we have to put We shall now introduce an approximation of a second kind. Let the velocity of the material particles be very small compared to that of light. Then will be the same as the time differential, . Further, , , will vanish compared to . We shall assume, in addition, that the gravitational field varies so little with the time that the derivatives of the by may be neglected. Then the equation of motion (for = 1,2,3) reduces to This equation is identical with Newton's equation of motion for a material particle in a gravitational field, if we identify with the potential of the gravitational field; whether or not this is allowable, naturally depends upon the field equations of gravitation, that is, it depends upon whether or not this quantity satisfies, to a first approximation, the same laws of the field as the gravitational potential in Newton's theory. A glance at (90) and (90a) shows that the actually do play the rôle of the intensity of the gravitational field. These quantities do not have a tensor character. Equations (90) express the influence of inertia and gravitation upon the material particle. The unity of inertia and gravitation [Pg 86] is formally expressed by the fact that the whole left-hand side of (90) has the character of a tensor (with respect to any transformation of co-ordinates), but the two terms taken separately do not have tensor character, so that, in analogy with Newton's equations, the first term would be regarded as the expression for inertia, and the second as the expression for the gravitational force. We must next attempt to find the laws of the gravitational field. For this purpose, Poisson's equation,