In slate-messages much use is made of a false flap, or a loose sheet of slate which fits imperceptibly on one side of the framed slate. It conceals the message written on the slate, and is removed under the table or under cover of a newspaper. A sheet of slate-coloured silk or cloth is sometimes fitted on the slate, and it is drawn up the medium's sleeve or rolled into the frame of the slate. Invisible messages may be written on the slate with onion or lemon juice, and developed by lightly passing over them a cloth containing powdered chalk. Double-frame slates lend themselves to infinite trickery. Slates are provided by "the trade" with false hinges and all kinds of mechanism. But even when the sitter brings his own slates, as Zöllner did, and ties them up and seals them, the medium is not baffled. They are laid aside, for the spirits to write on at their leisure. At the first convenient opportunity the medium removes the wax, without spoiling the seal, by passing a heated knife-blade or fine wire beneath it, and, after untying the strings, heats the under-surface of the wax and sticks it on again. Mediums found that sitters were greatly impressed if they heard the sound of the spirits writing on the slate. This was easily done by scraping with the finger nail, and cautious people wanted to have a security against fraud. One medium gave them adequate security. He held both hands above the table, yet writing was distinctly heard underneath it. The man had attached to the table a clamp holding a bit of slate-pencil, and against this he rubbed a pencil which was fastened to his trousers by loops of black silk. Others can use a pencil with their toes--I have seen an armless Bulgar girl use a pen with her toes as neatly as a good writer uses his fingers--and hold both hands above the table.