MIXTURES 213 



formed by heating white sand (silica, SiO 2 ) together 

 with a compound of each of the metals to be used. 

 Sand alone will not melt, but when heated with these 

 other compounds they all melt and unite together, 

 forming the silicates. This heated mass must then be 

 cooled slowly, to make the glass as tough as possible. 



The kind of glass formed, depends upon what metals 

 are used. Window glass is a silicate of Ca and Na; 

 bottle glass is a silicate of Ca, Na, Al, and Fe ; glass 

 used for lenses is a silicate of K and Pb. Very few sub- 

 stances act upon glass ; air does not affect it, and liquids 

 do not generally pass through it. For these reasons 

 it is very useful as material from which to 

 make bottles, jars, and other vessels. It is 

 also one of the very few transparent solids 

 that are common. Its importance is great. 



250. Wood. In general, wood is made up 

 mostly of cellulose ; the chief elements that 

 it contains are therefore C, H, and O. Mixed 

 with the cellulose are usually small quanti- 

 ties of mineral salts ; these are left as ashes 

 after wood is burned. Living wood always 

 contains some water ; and many kinds of FlG- 139 

 wood contain other substances, such as oils, acids, pitch, 

 resin, balsams, etc. 



The cellulose in wood usually occurs in the form of 

 fibers. In a cornstalk the fibers may be seen singly 

 (Fig. 139), but they are more commonly grouped 

 together in masses. The sap flows in spaces between 

 the masses of fibers. 



