Meaning of "Upon one other interest I have not yet touched—to me the weightiest and farthest—reaching of all"

Question

Arthur Conan Doyle's The History of Spiritualism contains the passage quoted below.

In his presidential address before the British Association at Bristol in 1898, Sir William briefly referred to his earlier researches. He said: Upon one other interest I have not yet touched—to me the weightiest and farthest—reaching of all. No incident in my scientific career is more widely known than the part I took many years ago in certain psychic researches. Thirty years have passed since I published an account of experiments tending to show that outside our scientific knowledge there exists a Force exercised by intelligence differing from the ordinary intelligence common to mortals . I have nothing to retract. I adhere to my already published statements. Indeed, I might add much thereto.

What does "one other interest I have not yet touched—to me the weightiest and farthest—reaching of all" mean? does it refer to a particular subject of presidential address?

Answer 1

The Wikipedia article about William Crookes points out that he

was named president of the British Association for the Advancement of Science in 1898.

His presidential address was his first speech to the association as a president. This address touched (up)on (i.e. discussed or mentioned briefly) several topics. From Wikipedia's point of view, the most important topic he discussed was the food shortage he thought would be coming without the production of nitrogen fertiliser. Arthur Conan Doyle is more interesting in another topic that Crookes discussed and that the scientist introduced with the words "Upon one other interest I have not yet touched — to me the weightiest and farthest-reaching of all".

This means that the topic of the existence of "a Force exercised by intelligence (...)" was the "most important" ("weightiest") topic in his speech and the one with the greatest influence ("farthest-reaching") on science. Looking back to this statement 130 years later, the implicit claim that spiritualism was a more important topic than a threat of food shortage may seem hard to take seriously, but that appears to be what he said.