136 PRACTICAL LESSONS IN SCIENCE. 



as carbonic acid, and yet the silicates generally are quite stable 

 compounds. Glass is manufactured from the silicates of sodium, 

 potassium, calcium, etc., with other ingredients. 



Silicon has been obtained as a brownish powder and in the 

 form of crystalline scales with metallic luster resembling graphite. 

 Carbon and silicon are similar in some respects, as both form 

 dioxides and weak acids, and the same hydrides and chlorides, 

 but differ in that most of the compounds of carbon are derived 

 from the hydrocarbons, while the compounds of silicon are derived 

 from silicic acid. 



Boron (B)is an element which has not been found in vegetable 

 or animal bodies ; is never found free ; is not abundant ; is not es- 

 sential to life, and yet in many ways it is an interesting element, 

 similar in some respects to carbon and silicon. It forms with 

 oxygen boric trioxide, B 2 3 , which uniting with water forms 

 boracic, or boric acid, H 3 B0 3 . 



This acid is found issuing from the earth as vapor, accom- 

 panied with steam in certain volcanic regions, as Northern Italy. 

 This steam is passed into water, which absorbs the acid yielding 

 boracic acid crystals on evaporation. Boracic acid is feeble like 

 silicic and carbonic acids. It fuses at high temperatures, when 

 it dissolves many of the metallic oxides forming glassy berates 

 which are of characteristic and often beautiful colors. On this 

 property depends the chief value of boron in the arts and in 

 chemistry. It forms a good test for detecting the presence of 

 metals, and is valuable in the manufacture of colored glass and 

 porcelains. It is used in hard soldering and in brass manu- 

 facture. The most important borate is the sodium borate called 

 borax, which is the form under which boron is best known. 



Boron has been isolated as an olive-green powder; also in 

 copper-colored scales similar to graphite; also in transparent 

 octahedral crystals, rivaling diamonds in brilliancy and hardness. 

 Carbon, silicon and boron seem to form a natural group; each 

 has a crystalline, amorphous and graphite-like form. Neither 

 can be made a vapor, and each is practically insoluble. 



