THE EARTHS. 137 



decay, and is evolved from various mines, volcanoes, and 

 other natural sources. From its grejit levity it has general- 

 ly been used to fill air-balloons. 



Water is said to be in a state of composition with other 

 bodies, because in many cases it becomes one of their com- 

 ponent parts. It is combined in a state of solidity in marble, 

 in crystals, in spars, in gems, and in many alkaline, earthy, 

 and metallic salts, both natural and artificial, to all of which 

 substances it imparts hardness, and to most of them transpa- 

 rency. Near the poles water is eternally solid ; there it is 

 similar to the hardest rocks, and may be formed by the chisel 

 of the statuary, like stone. It becomes still more solid in the 

 composition called mortar, and in cements, having parted 

 with more of its caloric in that combination than it does in 

 the act of freezing. If you take some ground plaster of 

 Paris, fresh calcined, and mix it with a little water, the affini- 

 ty of the plaster for the water is so great, that in a few minutes 

 the whole will be converted to a solid. 



Questions. — 1. Of what is water compoGcd ? 2. In what four 

 states is it found ? 3. What is its most simple state ? 4. What is the 

 difference between liquid water or vapour and ice ? 5. Why cannot 

 water in an open vessel be made hotter than its boiling point ? 6. How 

 may an animal be frozen to death in the midst of summer .'' 7. Why 

 would this happen ? 8. Explain the extinguishing of fire by water. 

 9. What space does vapour occupy .'' 10. What is said of the expan- 

 sive force of steam, and its probable application .-' 11. What is hydro- 

 gen, and how may hydrogen gas be obtained ? 12. What is the result 

 of kindling hydrogen gas on its rushing from the glass tube .' 13. 

 What is its weight and what space does it occupy ? 14. In what sub- 

 stances is water combined in a state of solidity ? 15. Why does water 

 become solid in mortar and in cements.'' [Note. Hydrogen (pron. 

 Hi'dro-jen,) takes its name from two Greek words signifying to pro- 

 duce water.] 



LESSON 63. 



The Earths and Alkalies. 



The earths are silex, or silica, alumine, glucine, zircon, yttria, 

 magnesia, barytes, strontites, and lime : — the four last mention-^ 

 ed are called alkaline earths. 



Stra'ta (plural of stratum) beds, layers. 



Earths are such incombustible substances as arc not 

 4uctile, are mostly insoluble ir* water or oil, and preserre 



12* 



