SILICON AND BOEON. 195 



pots, but would be decidedly so for most of the purposes to 

 which earthenware is applied. To remedy this defect the 

 ware is covered with a coat of glass. This glazing, as it is 

 called, is done in various ways. Common earthenware is 

 often glazed with oxide of lead. This is very dangerous if 

 the vessels are to be used in cooking or in preserving any 

 eatables, for the lead may be dislodged by some chemical 

 action of the contents, and act as a poison. Common salt 

 is also used. Being thrown into the kiln, it is raised in va- 

 por by the heat, and is decomposed on coining in contact 

 with the surface of the ware. The chlorine leaves the salt, 

 and its sodium becoming soda by attracting oxygen, the 

 soda unites with the silica of the ware and forms a glass. 

 For finer articles another mode is followed : A paste is made 

 of such materials as will, under the influence of powerful 

 heat, form a glass. These materials are reduced to an ex- 

 ceedingly fine powder, and this being diffused in water, the 

 article to be glazed is dipped into it. By this means it 

 gets a very thin coating of the glaze, for the clay absorbs 

 at once the moisture, and the fine powder remains uniformly 

 diffused over the surface. By intense heat this is converted 

 into a smooth coating of glass. The paste used is often 

 composed of feldspar, quartz, and borax. Glazing is not 

 necessary in the case of porcelain and some kinds of stone- 

 ware, for certain materials which form glass are mingled 

 with the clay, so that the heat of the baking fills up all the 

 minute spaces in the clay with glass. Still, the glazing is 

 usually done for the sake of adding to the beauty of the 

 ware. 



2C9. Boron. The element called boron is a gray amor- 

 phous powder. It is never found in nature, but the acid 

 which it forms, boracic acid, is sometimes exhaled from vol- 

 canic openings in the earth. The hot vapors of the lagoons 

 of Tuscany contain it in large quantity. In collecting it, 



