xiii THE DECOMPOSITION OF THE ALKALIS 283 



direct experiment that they were actually metallic oxides. 

 Although he was not able to determine their composition 

 by direct weighing, Davy satisfied himself: 



" That when the metals of the earths were burned in a 

 small quantity of air they absorbed oxygen, gained weight 

 in the process, and were in the highly caustic or unslacked 

 state ; for they produced strong heat by the contact of water, 

 and did not effervesce during their solution in acids." 



Lime was thus proved to be CALCIUM OXIDE. Slaked 

 lime, formed by combining lime with water, is sometimes 

 regarded as HYDRATE OF LIME, but is more usually described 

 as CALCIUM HYDROXIDE, i.e. a ternary compound of calcium, 

 hydrogen and oxygen. Similar names are given to the 

 other earths. 



Composition of the alkalis. Davy at first regarded the 

 alkalis as " highly combustible metallic bases united to 

 oxygen" (A.C.R. VI. 43.), but he soon discovered that the 

 oxides required to be combined with water in order to 

 convert them into caustic alkalis. He also confirmed the 

 observation of Gay-Lussac and Thenard that when the 

 metals were burned in oxygen, PEROXIDES were formed, 

 from which the excess of oxygen could be detached by 

 further heating, or by the action of water in converting the 

 peroxide into a common caustic. 



These relationships were demonstrated very clearly in his 

 experiments on the elementary nature of chlorine. He 

 combined chlorine gas (i) with metallic potassium, (2) with 

 the oxide and peroxide, (3) with the hydrated oxide (identi- 

 cal with ordinary caustic potash) and found that : 



(i) In the direct combination of potassium with chlorine 

 no water or oxygen was separated, the only product being a 

 neutral muriate or chloride formed by the direct combination 

 of about i -i cubic inch of chlorine with each grain of pot- 

 assium. 



