This may have been covered elsewhere but here is my attempt to explain what I regard as a major flaw/continuity error in the story. Lisle’s miraculous potion makes you live forever. That is one powerful potion. All of that magic – the ability to reverse aging and keep you living forever, and ever, in finitum - and it comes in a tiny little bottle about 4 inches high and less than an inch in diameter. To demonstrate the potion’s power, Lisle adds a drop of the potion to a cut in Ernest’s finger and it makes his hand ‘young’ – but Ernest ultimately declines the offer of eternal youth.
Problem number one: A potion of that size, one single drop must be pretty powerful. That’s got to be a few decades of youth right there, right? Wrong - it only ‘youths’ Ernest’s hand. Why?
Problem number two: Won’t Ernest’s hand now go on living for ever and ever?
Clearly, the way that the potion is taken is very important and it would seem that drinking it is the only way to get it into your whole system. Perhaps some kind of chemical reaction which you only get from swallowing it is needed for the potion to work, otherwise the effect is only partial, and apparently temporary given that Ernest ultimately dies. But perhaps not. Let’s consider the facts. Ernest lives to be a very old man of, what, 90 years old? Within that time he marries again, has several children, takes up hiking/mountaineering along with a whole host of other business and leisure pursuits, some of which (like the mountaineering) are pretty strenuous for a man of his age. But what if the drop of potion in his bloodstream didn’t make him live forever but instead revitalised him and gave him a longer life than he would have had otherwise? Perhaps the whole potion is required for that youth and vitality to extend to eternal life – like a jigsaw, all, not just one part, is needed.
As for Ernest’s hand, while it wouldn’t live forever for the reasons explained above, it is possible that it wouldn’t age. I imagine that during his life Ernest would have had to ‘disguise’ his unaging hand (as a trained surgeon/mortician, perhaps he used makeup to paint on liver spots or whatever), or else simply pass it off as a freak disfigurement or ‘just one of those things’. Either way, I can’t imagine it would have affected him in a major way.