How was it proved that $DNA$ is the genetic material?

(N/A) Although the discovery of nuclein by Meischer and the proposition of the principles of inheritance by Mendel occurred around the same time,it took a long time to discover and prove that $DNA$ acts as the genetic material.
By $1926$,the quest to determine the mechanism for genetic inheritance had reached the molecular level.
Previous discoveries by Gregor Mendel,Walter Sutton,Thomas Hunt Morgan,and numerous other scientists had narrowed the search to the chromosomes located in the nucleus of most cells.
Transforming Principle: In $1928$,Frederick Griffith,in a series of experiments with $Streptococcus$ $pneumoniae$ (the bacterium responsible for pneumonia),witnessed a miraculous transformation in the bacteria.
During his experiment,a living organism (bacteria) changed its physical form. When $Streptococcus$ $pneumoniae$ bacteria are grown on a culture plate,some produce smooth,shiny colonies $(S)$ while others produce rough colonies $(R)$.
This is because the $S$ strain bacteria have a mucous (polysaccharide) coat,while the $R$ strain does not.
Mice infected with the $S$ strain (virulent) die from pneumonia,but mice infected with the $R$ strain do not develop pneumonia.
$S$ strain $\longrightarrow$ Inject into mice $\longrightarrow$ mice die
$R$ strain $\longrightarrow$ Inject into mice $\longrightarrow$ mice live
Griffith was able to kill bacteria by heating them.
He observed that heat-killed $S$ strain bacteria injected into mice did not kill them.
$S$ strain (heat-killed) $\longrightarrow$ Inject into mice $\longrightarrow$ mice live
$S$ strain (heat-killed) $+ R$ strain (live) $\longrightarrow$ Inject into mice $\longrightarrow$ mice die
When he injected a mixture of heat-killed $S$ and live $R$ bacteria,the mice died. Moreover,he recovered living $S$ bacteria from the dead mice.
He concluded that the $R$ strain bacteria had somehow been transformed by the heat-killed $S$ strain bacteria. Some 'transforming principle' transferred from the heat-killed $S$ strain enabled the $R$ strain to synthesize a smooth polysaccharide coat and become virulent.
This must be due to the transfer of the genetic material. However,the biochemical nature of the genetic material was not defined by his experiments.