Oxygen does not always present itself in a gaseous state; it is a constituent part of a vast number of bodies, both solid and liquid, in which it exists in a much denser state than in the atmosphere; and from these bodies it may be obtained without much disengagement of caloric. It may likewise, in some cases, be absorbed from the atmosphere without any sensible production of light and heat; for, if the process be slow, the caloric is disengaged in such small quantities, and so gradually, that it is not capable of producing either light or heat. In this case the absorption of oxygen is called _oxygenation_ or _oxydation_, instead of _combustion_, as the production of sensible light and heat is essential to the latter. EMILY. I wonder that metals can unite with oxygen; for, as they are so dense, their attraction of aggregation must be very great; and I should have thought that oxygen could never have penetrated such bodies. MRS. B. Their strong attraction for oxygen counterbalances this obstacle. Most metals, however, require to be made red-hot before they are capable of attracting oxygen in any considerable quantity. By this combination they lose most of their metallic properties, and fall into a kind of powder, formerly called _calx_, but now much more properly termed an _oxyd_; thus we have _oxyd of lead_, _oxyd of iron_, &c. EMILY. And in the Voltaic battery, it is, I suppose, an oxyd of zinc, that is formed by the union of the oxygen with that metal? MRS. B. Yes, it is. CAROLINE. The word oxyd, then, simply means a metal combined with oxygen? MRS. B. Yes; but the term is not confined to metals, though chiefly applied to them. Any body whatever, that has combined with a certain quantity of oxygen, either by means of oxydation or combustion, is called an _oxyd_, and is said to be _oxydated_ or _oxygenated_. EMILY. Metals, when converted into oxyds, become, I suppose, negative? MRS. B. Not in general; because in most oxyds the positive energy of the metal more than counterbalances the native energy of the oxygen with which it combines. This black powder is an oxyd of manganese, a metal which has so strong an affinity for oxygen, that it attracts that substance from the atmosphere at any known temperature: it is therefore never found in its metallic form, but always in that of an oxyd, in which state, you see, it has very little of the appearance of a metal. It is now heavier than it was before oxydation, in consequence of the additional weight of the oxygen with which it has combined. CAROLINE. I am very glad to hear that; for I confess I could not help having some doubts whether oxygen was really a substance, as it is not to be obtained in a simple and palpable state; but its weight is, I think, a decisive proof of its being a real body. MRS. B. It is easy to estimate its weight, by separating it from the manganese, and finding how much the latter has lost. EMILY.