Citric Acid.
A London chemist was the inventor of citric acid, and, having his own prices as long as the way of making the acid was a secret, realized a large fortune.
This chemist trusted nobody, but worked entirely alone. He thought his secret very safe. It was necessary, however, to have a chimney to his laboratory, and chimneys sometimes want sweeping.
A rival, disguising himself as a chimney-sweep, got into the sanctum. He had all his eyes about him, as the saying is, and, when the chimney was swept, knew how to make citric acid, and thus a monopoly was ended.