See My Lawyer.

Robert Gardner and Billy Noble become interested in a machine capable--according to the inventor, Trueman--of producing artificial rubber. A trust is formed, circulars are sent out, and a demonstration is requested. When Billy discovers that the invention is fraudulent and refuses to demonstrate it, the trust lawyer becomes suspicious and notifies postal authorities. Robert feigns insanity, and Trueman accepts an offer of $1 million for the formula, which experts later discover produces an indestructible paving block.