THE ASH OF PLANTS. 125 
Soda, Na,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,O, H,O, 80, or 
Na 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. 
Buitis: cata, 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 kuown, being but little more than half as heavy as 
water. It burns with a vivid white light when heated in the air. 
Lithia, Li,O, 30, and its Hydrate, closely resemble the correspond- 
ing compounds of tle two elements above described. They yield by 
union with acids the lithia-salts. 
Rubidium, kb, 8.5, and Caesium, Cs, 133.—Besides Potas- 
sium, Sodium, and Lithiwm, 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, 
ALKaALI-EARTH Merrats. — 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 processes, 
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 
ight yellow color. In moist air it rapidly tarnishes and 
acquires a coating of lime 
