222 
Book II. 
REMARKS ON THE PRIMARY COMPOUNDS. 
ceptions, that all very combustible bodies are com 
pounds. 
Though the five classes into which primary com 
pounds have been divided in the preceding Chapters 
have been long known, and have always engaged the at 
tention of chemical philosophers, the number of indi 
viduals which belong to these classes has been very 
much encreased. About the commencement of the 18th 
century only the three alkalies, four of the earths, two 
of the oxides, five acids, and the compound combustibles, 
were known, and these but imperfectly. Six earths, 
3 oxides, and 28 acids have been added to the list of 
primary compounds by the labours of the philosophers 
of the 18th century. Nor ought we to forget that of 
these 36 new bodies, no less than 14 were discovered 
by the sagacity of the indefatigable Scheele.