According to Readers Guide to The Mahatma Letters to A. P. Sinnett:
Slade, Dr. Henry, an American medium of exceptional talents. He was thoroughly tested by HPB and HSO and was picked by HSO and others to go to Russia for scientific tests of his phenomena. At one time he gave special courses to professors of Leipzig University in Gemany, and as a result Prof. Zollner published a book Transcendental Physics in which he challenged the world to account for Slade's phenomena save by a "spiritual" hypothesis. The book was translated into English by C. C. Massey ... See bibliography HPB I: 525; ML index; ODL I: 201; SH, pp. 13, 18, 74, 166.
In 1881 Mme. Blavatsky wrote:
Slade is a man who seems to be surrounded with an aura, or magnetic atmosphere, capable of so saturating the objects about him as to make them subject to disintegration and reintegration at the caprice of some intelligent power which hears, consents, wills, and executes. He fancies it is the hovering soul of his deceased wife which, however, is believed to yield its place momentarily to other “spirits” to write their own messages to their own (surviving) friends, in their own languages—languages which neither Slade nor she ever knew. . . . Slade’s chief specialty is to get automatic writing upon slates under perfect test conditions; but he is also, sometimes, clairvoyant, has vaporous figures appear in the room, and under Professor Zöllner’s observation, he produced a series of novel and astounding phenomena illustrating the passage of matter through matter.
- "Huxley and Slade." The Banner of Light (October 28, 1876). Available at Blavatsky.net and reprinted in A Modern Panarion by H. P. Blavatsky.
- Milner, Richard. "Charles Darwin and Associates, Ghostbusters." Scientific American (October 1996). Available at Scientific American. Reposted with commentaries at Spiritualismlink.com.
- Milner, Richard. "Charles Darwin: Ghostbuster, Muse and Magistrate." Survival of the Fittest: Celebrating the 150th Anniversary of the Darwin-Wallace Theory of Evolution Issue 9 (2008). Journal of the Linnaean Society in London. Available at Darwinlive.com.
- Wikipedia has useful links, as well.
- George E. Linton and Virginia Hanson, eds., Readers Guide to The Mahatma Letters to A. P. Sinnett (Adyar, Chennai, India: Theosophical Publishing House, 1972), 244.
- Helena Petrovna Blavatsky, Collected Writings vol. III (Wheaton, IL: Theosophical Publishing House, 1995), 14-15.