When the gas holder is full it is buoyed up by the lighter gas, and occupies an elevated position, and as its supply is used up, the gas holder settles down into the water. In the operation of gas making many valuable secondary products are formed. The coal in the retorts is not entirely consumed, but is reduced to the condition of coke, and in this form is sold for fuel. The ammoniacal condensations are purified to form ammonia, while the coal tar, which but a few years ago was little more than a waste material, is now a valuable commercial product, being extensively used in the manufacture of the aniline, phenol, and naphthalene dyes, also in medicines and perfumes, and being used in crude form also as an important element in street paving compositions. _Water Gas._--In 1875 an important era in gas making was inaugurated by the introduction of what is known as “_water gas_,” so called for the reason that water in the form of steam is decomposed and its hydrogen, mixed with carbonic oxide gas, is mingled with a heavier carbon gas from oil, and is converted at a high temperature into a permanent, stable illuminating gas, at a much lower cost than coal gas. [Illustration: FIG. 223.--LOWE’S WATER GAS APPARATUS, PATENTED SEPTEMBER 21, 1875.] Fontana was the first to notice the decomposition of steam by incandescent carbon to form hydrogen and carbonic oxide. Ibbetson’s British patent, No. 4,954, of 1824, represents the first application of this principle. This was followed by Alexander Selligue, who, in 1834, obtained a French patent, No. 9,800, and in 1842 produced water gas at Batignolles, a suburb of Paris. Sanders’ United States patent, 21,027, July 27, 1858, was the basis of an experiment tried at the Girard House in Philadelphia. These, with Siemens’ British patents, Nos. 2,861, of 1856, and 972, of 1863, for methods of constructing furnaces, constitute the earlier steps in the development of water gas, although many other patents were granted prior to the latter date for various methods and forms of apparatus. The practical production and successful commercial use of water gas, however, began with T. S. C. Lowe, who obtained United States patent No. 167,847, September 21, 1875, and revolutionized the gas making industry. In less than a dozen years from the date of his patent 150 cities and towns in the United States were using water gas, and in 1886 the Franklin Institute gave to Mr. Lowe a grand medal of honor for his invention, which of those exhibited that year was believed to contribute most to the welfare of mankind by cheapening the cost of light. Fig. 223 represents an illustration of the Lowe apparatus as shown in his patent, and whose operation is as follows: Valves 9 and 10 being open, an anthracite coal fire in generator chamber 1 gives off carbonic oxide gas, which passes down pipe 2 and enters the base of superheater 3, where mixing with air coming down pipe 4, it burns to form an intense heat.