After all that has been said of it, the nineteenth century furnishes an ever-fresh and amazing retrospect. Within the memory of the living these now common facts—too vast and sublime, however, to be called familiar—were hid, with the great bulk of modern science, indeed, from the sages of the world. Oxygen had but just been discovered, a hundred years ago; hydrogen was unknown; water was supposed to be an elementary substance; fire and flame were mysteries; what the sun might be, and the nature of its light and heat, nobody could guess. After hydrogen had been found elsewhere, it was discovered (in 1781) that water is the result of its combustion with oxygen, and in 1805 that two parts in three of the vast volume of that element pervading and covering the earth are contributed by this ethereal ingredient. Several ways to dissociate the two gases were found, but the common and practical method was and is the contact of steam with red-hot carbon. This, in the absence of free oxygen, results in a transfer of the water oxygen to the carbon fuel in combustion, leaving the water hydrogen free. Red-hot iron answers a similar purpose, forming an oxide of iron (rust) in place of carbonic acid; but the consumption of so valuable an article as iron in the process has hitherto excluded this method from practical use, although there is now some prospect that by deoxidizing the iron-rust it may become available over and over for the elimination of hydrogen at a minimum of cost.
Until a recent date it has been quite generally taken for granted that, since to separate the two gases of water must cost as much heat as they will evolve by reuniting in combustion, there could be no possible profit in forcing the separation for the sake of fuel. Hence, the application of water hydrogen to practical purposes has been regarded as visionary. But there are some considerations on the other side also that seem to have been overlooked. The unavoidable waste in burning-solid fuel has been found to range from fifty per cent, as a minimum in the arts up to ninety-five per cent, as a common proportion in stoves, and thus to exceed by several volumes the whole cost of obtaining from water a gaseous fuel which can be used with but insignificant waste. Besides this, the doubted possibility of economizing the carbonic acid has also been realized, and that hitherto worthless incumbrance has been incidentally recarbonized in the process and utilized as carbonic oxide, to an economic success. Direct economies in the process have also been achieved, preventing great waste of heat in various ways, including that of a large amount hitherto lost in cooling off the finished gas. These recent—and American—improvements have suddenly given a practical character to the manufacture of water-gas, and a practical purpose to the elucidation of the subject.
Notwithstanding an unbroken succession of failures in the economic sense for more than half a century, the unlimited and ubiquitous stores of hydrogen "ore" have mightily stimulated inventors to the task of extracting treasure from these mines of fuel. Few objects have en-
