Does "law" here refer to a scientific law?
The first outstanding incident was the mediumship of the Eddy brothers, which has probably never been excelled in the matter of materialization, or, as we may now call them, ectoplasmic forms. The difficulty at that date in accepting such phenomena lay in the fact that they seemed to be regulated by no known law, and to be isolated from all our experiences of Nature. The labours of Geley, Crawford, Madame Bisson, Schrenck Notzing and others have removed this, and have given us, what is at the lowest, a complete scientific hypothesis, sustained by prolonged and careful investigations, so that we can bring some order into the matter.
The History of Spiritualism By A.C Doyle
XII: THE EDDY BROTHERS AND THE HOLMESES