Hershey and Chase studied bacteriophages which are viruses that attack bacteria. They labeled the bacteriophage with radioactive isotopes to see where it goes when a virus attacks. They discovered that nearly all the radioactivity was found in phosphorus, confirming that DNA contains genetic material, not protein.
Most scientists don't used the word law because that implies it is infallible but generally scientific laws and theories are used interchangeably so true.
