THE ASH OF PLANTS. 125 



Soda, Ta 2 O, 62. This alkali, the base of the soda salts, 

 is not distinguishable from potash by its sensible proper- 

 ties. 



Hydrate of Soda, or Caustic Soda, Na 2 O, H 2 O, 80, or 

 N"a H O, 40. This body is like caustic potash in appear- 

 ance and general characters. It forms soaps with the 

 various fats. While the potash-soaps are usually soft, 

 those made with soda are commonly hard. 



LITHIUM : RUBIDIUM : CAESIUM. 



I.itl&ium, Li, 7. The compounds of this metal are of much rarer 

 occurrence than those of Potassium and Sodium. The element itself is 

 the lightest metal known, being but little more than half as heavy as 

 water. It burns with a vivid white light when heated in the air. 



JLUlii;i, Li 2 O, 30, and its Hydrate, closely resemble the correspond- 

 ing compounds of the two elements above described. They yield by 

 union with acids the lithia-salts. 



Rubidium, Rb, 85.5, and Caesium, Cs, 133. Besides Potas- 

 sium, Sodium, and Lithium, there are two other recently discovered 

 alkali-metals, viz. : Rubidium and Caesium. These elements are com- 

 paratively rare, although they appear to be widely distributed in nature 

 in minute quantity. 



Rubidium has been found in the ashes of tobacco and sugar-beet, as 

 well as in commercial potash. Caesium, which is the rarer of the two, 

 has as yet not been detected in the ashes of plants, but undoubtedly oc- 

 curs in them. These metals and their compounds have, in general, the 

 closest similarity to the other alkali-metals. 



ALKALI-EARTH METALS. The two metallic elements 

 next to be noticed, viz. : Calcium and Magnesium, give, 

 with oxygen, the alkali-earths, lime and magnesia. The 

 metals are only procurable by difficult chemical pro-cesses, 

 and from their eminent oxidability are not found in nature. 

 They are but a little heavier than water. Their oxides are 

 but slightly soluble in water. 



CALCIUM AND ITS COMPOUNDS. 



Calcium, Ca, 40, is a brilliant ductile metal having a 

 light yellow color. In moist air it rapidly tarnishes and 

 acquires a coating of lime. 



