The incandescent-mantle gaslight takes advantage of the heat generated by the Bunsen flame and produces an incandescent light that has revolutionized gas lighting. The flame of the Bunsen tube is burned inside a mantle which is rendered incandescent by the heat. The incandescent mantle was invented by Dr. Auer von Welsbach and was known for a long time as the Welsbach light; but improvements in the process of making the mantles, brought other lamps of the same type on the market, when it became known as the mantle lamp. The first serviceable mantles were made in 1891 and from that time there has been a steady development in the gas-lighting industry. The original mantles were made of knitted cotton yarn, impregnated with rare earths and are still so made; but the most durable mantles are now constructed from ramie or china grass. After being knitted, the mantles are impregnated with thorium nitrate, with the addition of a small quantity of cerium nitrate, and occasionally other nitrates. The mantles are then shaped and mounted; the fiber is burned out and the mantles are dipped in collodion to give them stability for transportation. When placed in the lamp for use, the collodion is first burned off and the remaining oxide of thorium forms the incandescent mantle. One style of mantle is now being made in which the fiber is not burned out until it is placed in the lamp. They are commonly used with gasoline lamps and give very good results. [Illustration: FIG. 191.--Gas lamp with upright mantle.] The first incandescent-mantle gas lamps to be used were of the upright type, such as is shown in Fig. 191, and for a long time they were the only mantle lamps in use. While the upright mantle was a great improvement over the open-flame gas jet, the lamp was not satisfactory because of the shadows cast by the fixture and from the fact that a large amount of the light was lost by being directed upward from the incandescent mantle. With the development of the inverted type, the mantle lamp was greatly improved. In the use of lamps of any kind, the desired position of the illumination is that in which the light is directed downward. In the inverted type of mantle lamp this feature is accomplished and adds materially to the efficiency of the light, because the rays are sent in the direction of greatest service. The upright mantle lamps are still sold but by far the greater number offered for sale are of the inverted type.