#### Answer

The elastic force factor of a solid is significantly greater than that of air.

#### Work Step by Step

Comparing equations 11-13, 11-14a, and 11-14b, we see that the speed of a longitudinal wave is given by, roughly:
$$v=\sqrt{\frac{elastic force factor}{inertia factor}}$$
This is also stated on page 308.
Though the density of solids is typically thousands of times greater than that of air, the elastic force factor for a solid (i.e., its bulk modulus) is about a million times greater than it is for air. Thus, the speed of sound in most solids is greater than in air.