66 PRINCIPLES OF CHEMISTRY. 



Most, if not all, of the solid elements may be obtained in the crys- 

 tallized state; a few are amorphous and crystallized, or polymorphous. 

 The physical properties of many elements in these different states 

 differ widely. For instance : Carbon is known crystallized as diamond 

 and graphite, or amorphous as charcoal. The property of elements to 

 assume such different conditions is called allotropy, and the different 

 forms of an element are termed attotropic modifications. 



Some of the gaseous elements are also capable of existing in allo- 

 tropic modifications. For instance : Oxygen is known as such and as 

 ozone, the latter differing from the common oxygen both in its physi- 

 cal and chemical properties. The explanation given for this surprising 

 fact, that one and the same element has different properties in certain 

 modifications, is, that either the molecules or the atoms within the 

 molecules are arranged differently. Ozone, for instance, has thrce- 

 atoms of oxygen in the molecule, wliile the common oxygen molecule 

 contains but two atoms. 



Most of the elements are tasteless and odorless ; a few, however, 

 have a distinct odor and taste, as, for instance, iodine and bromine. 



Relationship between elements and the compounds formed 

 by their union. The properties of the compounds formed by the 

 combination of elements are so various that it is next to impossible 

 to give any general rule by which they may be indicated. It may 

 be said, however, that nearly all of the gaseous compounds contain 

 at least one gaseous element, and that solid elements, when combining 

 with each other, generally form solid substances, rarely liquids, and 

 never compounds showing the gaseous state at the ordinary tem- 

 perature. 



Nomenclature. The chemical nomenclature of compound sub- 

 stances has undergone considerable changes within the last twenty 

 years. These changes were made in conformity with our present 

 views of the constitution of the compounds. 



When two elements combine in one proportion only, little difficulty 

 is experienced in the formation of a name, as, for instance, in iodide 

 of potassium or potassium iodide, KI, chloride of sodium or sodium 

 chloride, NaCl. 



When two elements combine in more than one proportion, the 

 syllables, mono, di, tri, tetra, and penta are frequently used to designate 

 the relative quantity of the elements. For instance : Carbon mon- 

 oxide, CO, carbon dioxide, CO 2 , phosphorus tfn'chloride, PC1 3 , phos- 

 phorus pentachloiridQ, PC1 5 . 



