Elements alone, although not all of them, have the peculiar lustre, opacity, malleability, and the great heat and electrical conductivity which are proper to metals and their mutual combinations. But elements are far from all being _metals_. Those which do not possess the physical properties of metals are called _non-metals_ (or _metalloids_). It is, however, impossible to draw a strict line of demarcation between metals and non-metals, there being many intermediary substances. Thus graphite, from which pencils are manufactured, is an element with the lustre and other properties of a metal; but charcoal and the diamond, which are composed of the same substance as graphite, do not show any metallic properties. Both classes of elements are clearly distinguished in definite examples, but in particular cases the distinction is not clear and cannot serve as a basis for the exact division of the elements into two groups. The conception of elements forms the basis of chemical knowledge, and in giving a list of them at the very beginning of our work, we wish to tabulate our present knowledge on the subject. Altogether about seventy elements are now authentically known, but many of them are so rarely met with in nature, and have been obtained in such small quantities, that we possess but a very insufficient knowledge of them. The substances most widely distributed in nature contain a very small number of elements. These elements have been more completely studied than the others, because a greater number of investigators have been able to carry on experiments and observations on them. The elements most widely distributed in nature are:-- Hydrogen, H = 1. In water, and in animal and vegetable organisms. Carbon, C = 12. In organisms, coal, limestones. Nitrogen, N = 14. In air and in organisms. Oxygen, O = 16. In air, water, earth. It forms the greater part of the mass of the earth. Sodium, Na = 23. In common salt and in many minerals. Magnesium, Mg = 24. In sea-water and in many minerals. Aluminium, Al = 27. In minerals and clay. Silicon, Si = 28. In sand, minerals, and clay. Phosphorus, P = 31. In bones, ashes of plants, and soil. Sulphur, S = 32. In pyrites, gypsum, and in sea-water. Chlorine, Cl = 35·5. In common salt, and in the salts of sea-water. Potassium, K = 39. In minerals, ashes of plants, and in nitre. Calcium, Ca = 40. In limestones, gypsum, and in organisms. Iron, Fe = 56. In the earth, iron ores, and in organisms. Besides these, the following elements, although not very largely distributed in nature, are all more or less well known from their applications to the requirements of everyday life or the arts, either in a free state or in their compounds:--