And cannot the oxyds of gold, silver, and platina, which are formed by means of acids or of the electric fluid, be restored to their metallic state? MRS. B. Yes, they may; and the intervention of a combustible body is not required; heat alone will take the oxygen from them, convert it into a gas, and revive the metal. EMILY. You said that rust was an oxyd of iron; how is it, then, that water, or merely dampness, produces it, which, you know, it very frequently does on steel grates, or any iron instruments? MRS. B. In that case the metal decomposes the water, or dampness (which is nothing but water in a state of vapour), and obtains the oxygen from it. CAROLINE. I thought that it was necessary to bring metals to a very high temperature to enable them to decompose water. MRS. B. It is so, if it is required that the process should be performed rapidly, and if any considerable quantity is to be decomposed. Rust, you knew, is sometimes months in forming, and then it is only the surface of the metal that is oxydated. EMILY. Metals, then, that do not rust, are incapable of spontaneous oxydation, either by air or water? MRS. B. Yes; and this is the case with the perfect metals, which, on that account, preserve their metallic lustre so well. EMILY. Are all metals capable of decomposing water, provided their temperature be sufficiently raised? MRS. B. No; a certain degree of attraction is requisite, besides the assistance of heat. Water, you recollect, is composed of oxygen and hydrogen; and, unless the affinity of the metal for oxygen be stronger than that of hydrogen, it is in vain that we raise its temperature, for it cannot take the oxygen from the hydrogen. Iron, zinc, tin, and antimony, have a stronger affinity for oxygen than hydrogen has, therefore these four metals are capable of decomposing water. But hydrogen having an advantage over all the other metals with respect to its affinity for oxygen, it not only withholds its oxygen from them, but is even capable, under certain circumstances, of taking the oxygen from the oxyds of these metals. EMILY. I confess that I do not quite understand why hydrogen can take oxygen from those metals that do not decompose water. CAROLINE. Now I think I do perfectly. Lead, for instance, will not decompose water, because it has not so strong an attraction for oxygen as hydrogen has. Well, then, suppose the lead to be in a state of oxyd; hydrogen will take the oxygen from the lead, and unite with it to form water, because hydrogen has a stronger attraction for oxygen, than oxygen has for lead; and it is the same with all the other metals which do not decompose water. EMILY. I understand your explanation, Caroline, very well; and I imagine that it is because lead cannot decompose water that it is so much employed for pipes for conveying that fluid. MRS. B.