The elementary substances of which a body is composed are called the _constituent_ parts of that body; in decomposing it, therefore, we separate its constituent parts. If, on the contrary, we divide a body by chopping it to pieces, or even by grinding or pounding it to the finest powder, each of these small particles will still consist of a portion of the several constituent parts of the whole body: these are called the _integrant_ parts; do you understand the difference? EMILY. Yes, I think, perfectly. We _decompose_ a body into its _constituent_ parts; and _divide_ it into its _integrant_ parts. MRS. B. Exactly so. If therefore a body consists of only one kind of substance, though it may be divided into its integrant parts, it is not possible to decompose it. Such bodies are therefore called _simple_ or _elementary_, as they are the elements of which all other bodies are composed. _Compound bodies_ are such as consist of more than one of these elementary principles. CAROLINE. But do not fire, air, earth, and water, consist, each of them, but of one kind of substance? MRS. B. No, my dear; they are every one of them susceptible of being separated into various simple bodies. Instead of four, chemists now reckon upwards of forty elementary substances. The existence of most of these is established by the clearest experiments; but, in regard to a few of them, particularly the most subtle agents of nature, _heat_, _light_, and _electricity_, there is yet much uncertainty, and I can only give you the opinion which seems most probably deduced from the latest discoveries. After I have given you a list of the elementary bodies, classed according to their properties, we shall proceed to examine each of them separately, and then consider them in their combinations with each other. Excepting the more general agents of nature, heat, light, and electricity, it would seem that the simple form of bodies is that of a metal. CAROLINE. You astonish me! I thought the metals were only one class of minerals, and that there were besides, earths, stones, rocks, acids, alkalies, vapours, fluids, and the whole of the animal and vegetable kingdoms. MRS. B. You have made a tolerably good enumeration, though I fear not arranged in the most scientific order. All these bodies, however, it is now strongly believed, may be ultimately resolved into metallic substances. Your surprise at this circumstance is not singular, as the decomposition of some of them, which has been but lately accomplished, has excited the wonder of the whole philosophical world. But to return to the list of simple bodies--these being usually found in combination with oxygen, I shall class them according to their properties when so combined. This will, I think, facilitate their future investigation. EMILY. Pray what is oxygen? MRS. B.