What does the older man talk about as he and William stroll through the garden in the story, Kew Gardens?

Accompanied by William who steers him through a garden course, this older man smiles and talks "almost incessantly" to himself. To William he speaks of "spirits of the dead" and their experience in Heaven. He tells William of an electrical contraption that would allow a widowed woman to talk to the spirits in Heaven; the older man then gets distracted by the "purple black" dress of a woman who he seemingly mistakes as a widow. William distracts the older man by drawing attention to a flower, to which the confused older man listens momentarily before changing the subject to the "forests of Uruguay, which he had visited hundreds of years ago in company with the most beautiful woman in Europe."