18. Let $S _ { n }$ denote the sum of the first $n$ terms of $\left\{ a _ { n } \right\}$. Given that $a _ { n } > 0 , a _ { 2 } = 3 a _ { 1 }$, and the sequence $\left\{ \sqrt { S _ { n } } \right\}$ is an arithmetic sequence. Prove that $\left\{ a _ { n } \right\}$ is an arithmetic sequence.