Hodgson more than suspected him, and he at last found a man, Mr. S. J. Davey, who was able to reproduce all his tricks. He wrote messages while he held the slates under the table, and he substituted prepared slates for clean slates under the noses of his sitters. Perhaps the most valuable part of his experience was this substitution, which is one of the fundamental elements of mediumistic trickery. Spiritualists--indeed, inquirers generally--honestly flatter themselves that they have taken care that there was no deception of this kind. Such confidence is foolish, as the professional conjurer does this kind of substitution under our eyes habitually, and we never see him do it. In order to make people more cautious Davey, with Dr. Hodgson's connivance, set up as a medium and gave sittings to Spiritualists. They afterwards sent accounts of their experiences to the Society for Psychical Research. They were, as usual, certain that there was no trickery, and that the messages were genuine. Davey then wrote correct accounts of what he had done, and it was seen that the accounts of the sitters were inaccurate and their observation faulty. Some of them indignantly retorted that Davey was a genuine medium, but found it more profitable to pose as a conjurer and exposer of mediums! In a work specially devoted to this subject (_Spirit Slate Writing and Kindred Phenomena_, 1899) Mr. W. E. Robinson gives about thirty different fraudulent ways of getting spirit-messages. Indeed, many of these may be sub-divided, and you get scores of methods. One method, for instance, is to write a message with invisible fluid on paper, seal the apparently blank paper in an envelope, and then let the message appear and pretend that the spirits wrote it. Mr. Robinson gives thirty-seven different recipes for the "invisible ink," and sixteen of these require only heat, which is easily applied, to develop them. In other cases the inside of the envelope has been moistened with a chemical solution which develops the hidden writing. One medium used to put an apparently blank sheet of paper in a clear bottle and seal it. Here trickery seemed impossible, and the sitter was greatly impressed at receiving a pious message on the paper. But the message had been written in advance with a weak solution of copper sulphate, and the bottle had been washed out with ammonia, which develops it.