CHEMICAL PROPERTIES OF CLAY. 41 



Of those mentioned in the above list, carbon and sulphur are 

 the only ones ever found in the elementary condition in clays. 

 The others are usually found in combination with each other. 

 Thus, for example, silicon unites with oxygen to form the com- 

 pound known as silica, which consists of one atom, of silicon and 

 two atoms of oxygen, and which would be designated by the 

 symbol SiO s . Similarly, two of aluminum will unite with three of 

 oxygen, forming the compound known as alumina and represented 

 by the symbol A1 2 3 ; again, iron in similar combination may give 

 either FeO or Fe 2 O s ; or CaO (lime) may be formed from 

 calcium and oxygen. Carbon and oxygen form C0 2 , known as 

 carbon dioxide or carbonic acid gas. If the latter unites with 

 CaO, we get a compound expressed by the symbol CaC0 3 , and 

 called lime carbonate; CaO and Si0 2 may unite, giving CaSi0 3 , 

 which is called a silicate of lime, because it is a compound con- 

 taining calcium, silicon and oxygen. 



The elements are divisible into two groups, the one known 

 as acid elements, the other as basic elements or bases. The latter 

 are commonly oxides of the metallic elements, and include CaO 

 (lime), MgO (Magnesia), A1 2 3 (alumina), Fe 2 3 (ferric 

 oxide), K 2 (potash), Na 2 (soda). The acids and bases 

 are strongly opposed in their characters, and, while there is little 

 or no affinity between members of the same group, those of 

 opposite groups show a marked affinity for each other. An acid, 

 therefore, tends to unite with a base under favorable conditions, 

 these conditions being either the presence of moisture or heat, 

 both of which promote chemical activity and combination. Com- 

 pounds formed by the union of acid elements and basic elements 

 are termed salts, and the different ones possess a different degree 

 of permanence or destructibility. Thus, some exist only at 

 low temperatures, and are broken up or pass off in gaseous 

 form at a red heat, while others may form only at a temperature 

 of redness or higher. 



oceans, and those portions of the original rocks which projected above the 

 ocean were attacked by the weather, in the same manner as described on 

 pages 4-6, the products of rock decay being washed down into the seas, 

 where they were deposited as sediments. The elements of the original rocks 

 would, therefore, be found partly in these sediments and partly in solution 

 in the sea water. 



