A hundred years ago, scientists knew that chromosomes were made up of DNA and proteins. At this time a new dye was developed that could bind specifically to DNA and that stained cell nuclei red in direct proportion to the amount of DNA present in the cell. This technique provided circumstantial evidence that DNA was the genetic material:
DNA was in the right place. DNA was confirmed to be an important component of the nucleus and the chromosomes, which were known to carry genes.
DNA was present in the right amounts. The amount of DNA in somatic cells (body cells not specialized for reproduction) was twice that in reproductive cells (eggs or sperm)—as might be expected for diploid and haploid cells, respectively.
DNA varied among species. When cells from different species were stained with the dye and their color intensity measured, each species appeared to have its own specific amount of nuclear DNA.
You should already realize that circumstantial evidence is not a scientific demonstration of cause and effect. After all, proteins are also present in cell nuclei. Science relies on experiments to test hypotheses. The convincing demonstration that DNA is the genetic material came from two sets of experiments, one with bacteria and the other with viruses.