Parmenides had already declared that taste, colors, sound and the like were only names, standing for no objective qualities. Democritus agreed that sensations _represent_ nothing external to ourselves, though they are _caused_ by something outside us. For example, what we call pungency, tartness, bitterness, saltiness, etc., were impressions produced on the palate by atoms of a certain shape--sharp, rough, pointed or hooked particles of matter producing an effect of pungency or acidity. Atoms with smooth surfaces form substances which ordinarily impress the senses agreeably. From this we learn the effect upon our palates of certain substances, but these sensations tell us nothing of the true nature of the substances tasted. “By the senses,” says Democritus, “we in truth know nothing sure, but only something that changes according to the disposition of the body and of the things that enter into it or resist it” (Fragment 9). True reality lies beyond sense impressions; “truth is in the depths” (Fragment 11). Our ideas represent our impressions, and are not direct reproductions of the external objects themselves, “the inner essence of which is concealed from us.” This ancient theory of the differences of taste depending on differences in the shapes--or roughness or smoothness--of the particles composing substances prevailed even well into the eighteenth century of our own era; dictionaries of the period still defining acidity, for example, as due to sharp, pointed particles. Today we still admit that atoms are qualitatively homogeneous--but by “atoms” we now mean electrons, the constituent electrical charges which make up the “atoms” of the chemist. Differences in the number of electrons in atoms confer upon them qualitative (chemical) differences, though all atoms of any one element are chemically alike--are homogeneous. Differences in the number of spatial arrangement of these atoms (in groups or “molecules”) constitute both physical and chemical differences in substances, i.e., in compounds. Quite different substances are produced by combinations of precisely the same _kinds_ of atoms, but in different proportions. Take from a molecule of certain substances one single atom, and they may be changed from a compound necessary to life and growth into a deadly poison. Phosphorus is an element, and thus contains but one kind of atoms; but some (common) phosphorus is yellow and some (amorphous) is red, varying with the spatial distribution of the atoms in the molecules composing the phosphorus.