"The retorts are set in a framework of brick, and look like small ovens. The coal is put in with a long shovel, and after the retort is closed it is baked four or five hours, when it is drawn out and replaced by a fresh charge. "The gas goes from the retorts to the purifier, which consists of a series of pipes surrounded by water. It travels through these pipes till it is thoroughly cooled and gives up the tar and other impurities contained in it; then it passes through water and water-spray, to wash away ammonia, another impurity; next it is forced through powdered lime, to remove the sulphur contained in it; and then it goes to the gasometer, whence it is carried in pipes to the places where it is to be consumed." [Illustration: DIAGRAM OF GAS-WORKS.] "That's the whole story," responded Frank; "and I am going to make gas on a small scale to amuse the king. We cannot make our gas as pure as it is made in a large establishment, but we'll get it up so as to answer our purpose." With this understanding the boys went to work, and before night they had accumulated most of the materials needed for their performance. From one of the boxes Frank took a coil of rubber pipe and a slender "drop-light," which he had brought along, with the consent of the Doctor. A reflector, to be placed on this burner, was made by cutting an empty provision-can so as to form a cone, and carefully cleaning the surface of the tin on the inside. A small hand-mirror was mounted on a pivot, so that it could be used for turning the light on any desired point, and another mirror was arranged to be hung in front of the light and rotated at will. Half a dozen jars, with narrow mouths and covers to fit, were obtained by the efforts of Abdul, and also a quantity of soft clay, for closing them hermetically when desired. A couple of old gun-barrels were bought from a native, to serve as tubes to carry the gas from the retorts to the tub of water which was to serve as a purifier. A jar placed in this water with its mouth downward was the gasometer, or receiver, and then the apparatus was pronounced complete. "But how'll you manage to take your gas from the receiver to the king's palace?" said Fred. "Oh, that's easy enough," was the reply. "You know we have a lot of rubber bags for carrying things in and preserving them from the moisture of the climate. We will make a small hole in the gas-receiver, and fill the bags one after the other by placing them over this hole, which we can plug with a cork when we want to close it. "We can tie the mouths of the bags tight enough to prevent much loss, and in this way carry the gas to the palace. The rubber tube will make the connection from the bag to the burner. We can get sufficient pressure by having a man sit on the bag while we are using the light; and when we exhaust a bag, and want to replace it with a fresh one, we can easily make some excuse for suspending operations a few moments."