78 Introductory Biology 



Certain colloids, known as emulsoids (L. emulgere, to drain), are 

 less selective regarding their adsorption of ions and tend to adsorb mole- 

 cules of the medium in which they are dispersed (dispersion medium). 

 As these colloidal particles adsorb these molecules of the dispersion 

 medium, they may swell until the entire colloidal system becomes more 

 and more viscous, or even semisolid, as in jellies, gelatin desserts, etc. The 

 swelling of dried fruit in water is another example. 



In general, colloids are of great importance because all vital processes 

 of animals and plants are associated with colloidal materials. The liv- 

 ing protoplasm of plants and animals is colloidal in character. Many 

 foods of animals are colloids. The growth of plants, the germinating of 

 seeds, and many similar phenomena are associated with colloids and 

 their properties. 



Matter, Atoms, Molecules, and Elements 



Matter, of which all materials are composed, is made of extremely 

 small, microscopically invisible molecules (L. molecula, little mass) with 

 intermolecular spaces between them. A molecule is composed of the union 

 of two or more atoms and is the smallest unit of matter capable of a 

 separate, distinct physical existence. A molecule of free oxygen consists 

 of two atoms (Figs. 25 and 26). A molecule of water consists of two 

 atoms of hydrogen and one atom of oxygen; hence the molecular formula 

 for water is H2O (Fig. 27). An atom (Gr. atomos, indivisible) is the 

 smallest particle of an element capable of taking part in a chemical reac- 

 tion. There are as many different kinds of atoms as there are elements 

 (elementary substances), and vice versa. An element is composed of 

 atoms having the same atomic number, and, by ordinary means, cannot 

 be built up from simpler or decomposed into simpler substances. There 

 are over ninety definitely known basic elements, with claims for the dis- 

 covery of a few more. The chemical elements are known by symbols 

 which usually are the first or first few letters of the name of the element. 

 For example, the symbol for hydrogen is H; for oxygen, O; for carbon, 

 C; for magnesium, Mg.; etc. In some cases the symbol is derived from 

 the Latin name of the element. For example, the Latin name for iron 

 is ferrum, and the symbol is Fc; the Latin name for potassium is kalium, 

 and the symbol is K; the name for sodium is natrium, and the symbol is 

 Na.; etc. All the known elements are grouped in a table (periodic ar- 

 rangement of the elements based on their atomic structure) and each 

 element is given a specific number. Hydrogen has the atomic number 1 ; 

 helium, 2; carbon, 6; nitrogen, 7; oxygen, 8; magnesium, 12; mercury, 

 80; uranium, 92; etc. 



