Now, to decompose water it is only necessary to act upon the principle of hydrogen being a combustible substance. All substances are not equally combustible; that is to say, they do not burn or combine with oxygen with equal facility or quickness. Thus charcoal is more combustible than iron, iron is more combustible than copper, and copper than gold or silver, whilst phosphorus is still more combustible than charcoal. Now, oxygen will combine with any of these combustible substances; but if it have a choice, it will take that which is most combustible--that which it likes best. And even if the oxygen be already united with one body, and that another more combustible be brought into action on it, it will leave the former, and attach itself altogether to the latter substance. The combustibility of hydrogen is about equal to that of iron. It is inferior to carbon and to many other bodies; but it is superior to that of copper, silver, gold, and others. If, therefore, we take water in the state of steam, and bring it into contact with red-hot charcoal or coke, the oxygen of the water goes to the most combustible body, and the hydrogen is set free. In this way charcoal may be made to burn brilliantly without air, but not without oxygen. A red-hot bit of charcoal burns in steam, because it decomposes the water; it takes the oxygen, and turns the hydrogen out, which assuming the form of gas, may be collected by means of peculiar chemical apparatus. Iron and hydrogen are, as mentioned above, about equally combustible: in fact it depends upon the degree of heat, which is the more combustible. If the iron be bright red, it decomposes water, taking away the oxygen; but if it be only dull red, then hydrogen is the more combustible; and if there be a compound of oxygen and iron ready formed (oxide of iron, rust), the hydrogen will decompose it, and water being formed, the iron will be set free. If, therefore, a gun barrel be laid across a fire, and heated to bright redness, and a little water be poured into it at one end by means of a tun-dish with a stop-cock soldered to it, hydrogen gas will issue from the other end, and may be burned, or collected for various purposes.