At the time, blood transfusions were a new and exciting medical science, to the point where doctors were making some outrageous claims about what they could achieve with it. Thornton's belief was that by warming Washington's corpse up by the fire and injecting it with lamb's blood, he could essentially wind the former President back up again like a worn-down pocket watch. Of course, this is laughable based on what we know about science today. But even at the time, it seems a little odd that they skipped every testing phase that you'd expect to come before "zombify the ex-president."
Like "shut his tomb door in case this bullshit works."
Unfortunately, Washington's widow Martha rejected the plan to bring her husband back and the probable brain-eating rampage that would follow. Thornton backed down, but spent the next 20 years hating himself for not going against her wishes and doing it anyway. To be honest, we think history is poorer for it, too. Hell, think of the monuments we'd have today if the father of America's final act had been to chew open the skulls of his loved ones before his maid was forced to hack him to pieces with an ax.