244 ESSAYS snp OBSERVATIONS © 
earths and acids than between thefe and 
magnefia: But how does it then happen, 
that, if maznefia faturated with air be 
mixed with a compound of acid and cal- 
carious earth, thefe two laft, which attraét 
one another the moft ftrongly, do not re- 
main united ; but the acid is joined to the 
magnefia, and the calcarious earth to the 
air which it attraéts much more weakly 
than it does the acid? Is it becaufe the 
tum of the forces which tend to join the 
magnefia to the acid and the calcarious 
earth to the air, is greater than the fum of 
the forces which tend to join the calcari- 
ous earth to the acid, and the magnefia to 
the air; and becaufe there is a repulfion 
between the acid and air, and between 
the two earths; or they are fome how 
kept afunder in fuch a manner as hin- 
ders any three of them from being united 
together? 
THE firft part of this fuppofition is fa- 
youred by our experiments, which feem 
to fhew a greater difference between the 
forces wherewith the calcarious earth and 
ynagnefia attract fixed air, than between 
_ thofe 
