General Principles. 55 



called the mineral alkali, because it was found 

 in some mines, especially in Egypt. That 

 which is used in commerce is obtained from 

 the ashes of certain marine plants, and is known 

 in commerce under the name of barilla. 



These two have been very lately decomposed 

 by Professor Davy, and are found to be the hy- 

 d rated oxides of peculiar metals, the nature of 

 which is, however, as yet but very little known ; 

 and they may (as to the metallic bases at least) 

 be still classed among simple substances. The 

 third, ammonia, has been long ascertained to be 

 a compound of nitrogen and hydrogen ; an ac- 

 count of it will therefore come properly under 

 the class of Alkaline Salts, when some further 

 observations will be introduced on the fixed 

 alkalies. 



X. XI. The earths and metals will require 

 distinct lectures ; and being substances of a pe- 

 culiar character, and not entering so copiously 

 into the composition of those bodies which are 

 found on the surface of the earth as those I have 

 been describing, any observations on their na- 

 ture or combinations would be foreign to the 

 object of this lecture. 



I have thus endeavoured to lay before you a 

 general outline >f the doctrines of modern che- 

 mistry. I fear this lecture will appear unin- 

 teresting to many of you. It is, however, of 

 so much importance that, if it is well under- 

 stood, it will greatly facilitate the study of all 



