a 4 3 ON A NATUHAL SYSTEM 



five diflinft genera. It is very rare, if ever, that 

 they are found in n fimple ftate, being either 

 combined with one or more of the other earths. 

 The moil ealy method, therefore, would be to de- 

 termine the genus of every fuch compofition, 

 according to the heaviell principle ; but the 

 cafes before ieparately ilated, in 5 Ixuii are often 

 objections to this plan. 



LXXVIII. Exceptions. 



WERE this rule once admitted, we fliculd lofe 

 altogether the magnellan and argillaceous ge- 

 nera ; fur, in the compolhious hitherto examin- 

 ed, into which thofc earths enter, the filiccous 

 lias been always found to outweigh the others, 

 although, from their character and properties, 

 they hud both the fupcriority. Common clay 

 contains above half its weight of iilicccus earth, 

 ibmetimes above three fourths, and yet the ar- 

 gillaceous qualities arc fo dirtincl, that thele com- 

 petitions are unanimously denominated argilla- 

 ceous. The fume richnefsund pre-eminence of 

 quality, with rcfpccl to the filiccous earth, are 

 found in magnciia, and oilier fubflances. 



All earthy competitions, tlierefore, may be 

 determined by the genus cf tliat ingredient, 

 which exceeds the others in weight, unlcfs ir be 

 flliceous, and not equal to fevcn-eights ot the 

 whole. In fuch cafes, the genus ought to be 



afcertained 



