#### Answer

When the volume of the gas in Figure19-1. is doubled, it is equivalent to multiplying the length of the box L by 2 in Figure19-3a. The gas in Figure 19-1. tends to expand to fill all the available volume.
A second explanation for the gas expansion is that the system energy tends to be distributed among the greater possible number of available energy levels in the 2L box compared to its counterpart, the L box.