Clam and Tomato Bisque

Pour water over clams, then drain. To water add hard part of clams finely chopped. Heat slowly to boiling-point, cook twenty minutes, then strain. Cook butter with onion five minutes; remove onion, add flour and gradually clam 133water. Add cream, soft part of clams, and as soon as boiling-point is reached, tomatoes to which soda has been added. Season with salt and cayenne, and serve at once.