LESSONS IN CHEMISTRY. 135 



marks of the hydrocarbons. The ordinary illuminating gas 

 consists of free hydrogen, marsh gas, olefiant gas, carbonic 

 oxide, acetylene and other substances. 



SILICON (Si), next to oxygen, is the most abundant element in 

 nature. It has never been formed in an uncombined state, occur- 

 ring most abundantly as silicon dioxide or silica, formula, Si0 2 . 

 The dioxide uniting with water forms silicic acid, H 2 Si0 3 . This 

 acid forms silicates of sodium, potassium, aluminium, calcium 

 and other metals. These silicates are very abundant, silica and 

 the silicates make up the greater part of the crust of the earth. 

 The granite rocks, volcanic rocks, sand rocks, clay or slate rocks 

 everything except limestone and coal is composed largely of 

 the compounds of this element. Silica is not only abundant and 

 useful but is beautiful as well. The purest natural variety is the 

 transparent, colorless quartz crystal or rock crystal, occur- 

 ring in six-sided prisms terminated by six-sided pyramids. They 

 may be distinguished from most other crystals not only by their 

 form, but by their hardness, scratching glass almost as readily 

 as the diamond. With a little tinge of purple, the rock crystal 

 becomes the beautiful amethyst; when brown or smoky, Scotch 

 pebbles. Losing its transparency and crystalline form, we have 

 the translucent chalcedony, the brilliant carnelian, the banded 

 agate, the catseye, the onyx and the opal. Each of these forms 

 beautiful and enduring. Flint is a dark colored variety of silica 

 which is proverbial for its hardness. The percussion of flint and 

 steel for many years served our forefathers as a source of light 

 and heat in place of the lucifer match. 



While silica is insoluble, some of its compounds are soluble, 

 and plants contain considerable quantities in their composition, 

 and animals as well. Many natural waters contain silica in solu- 

 tion, as in the Geysers of Iceland, and petrified wood indicates 

 that silica is soluble to some extent in water, but probably only 

 in hot water and under considerable pressure. 



Hydrofluoric acid is the only substance that can decompose 

 silica at ordinary temperatures. Silicic acid does not appear to 

 be a very strong acid, seeming to be of about the same strength 



