How did the chemical nature of the ‘transforming principle’ get established?

Griffith was not able to find the chemical nature of transforming agent. It was finally Hershey and Chase who proved that DNA is the genetic material. They proved it by using radio labelled elements, such as ${}^{35}S$ and ${}^{32}P$. Radioactivity was detected in the cell which was due to the presence of ${}^{32}P$ in DNA.