In present-day English, the verb will is typically used as an auxiliary verb, either to express a future action or a habit; in these cases, it accompanies another verb.
In this passage from Doyle's The History of Spiritualism, "will" is used as a transitive verb. For example, as in the phrase "He willed me to do it", which means "He strongly wanted me to do it". However, in the context of the above passage from Doyle's book, "will" has a slightly different implication, since you cannot simply tell or convince a substance (the "ectoplasmic forms"; cf. ectoplasm to do something or to move in a move in a specific direction.
What Olcott is describing (according to Doyle) is that a person ("a strong-minded sitter") used their mind to move those substances; in other words, that person "willed" those substances to move. This is obviously only possible if you believe in telekinesis or psychokinesis.