3 6 Mr. kir wan’s Experiments and Obfervatlons cn 
may be faid to have a drong affinity to each other, and it is 
thus that acids unite to alkalies, earths, and metals, for the 
mod part. 
In order to determine the degrees of affinity, Mr. geoffroy 
has laid it down as a general rule, that when two fubdances 
are united, and either quits the other to unite to a third, that 
which thus unites to this third fubftance mull be faid to have a 
greater affinity to it than to the fubftance it has quitted. This- 
undoubtedly is the cafe when only two attractive powers are 
concerned ; thus, when felenite isdecompofed by a cauftic fixed, 
alkali, it is evident, that the vitriolic acid has a ftronger affi- 
nity to the alkali than to the earth ; but in many cafes a de- 
composition, feemingly tingle, is in fadt double, and the refult 
of the adtion of more than two powers, and then it is not eafy 
to know which is the greateft, nor confequently to determine 
the degree of attraction ; for in dance, the vitriolic acid unites 
to a mild fixed alkali, and expels the fixed air from it, yet it 
does not neceffiarily follow, that the vitriolic acid attradls, or is- 
attracted, by the alkali more drongly than the aerial acid ; for 
though there appears here only a lingle decompofition, yet in 
reality a fort of double decompofition takes place, the vitriolic 
acid giving out its fire to the aerial, while the aerial religns the 
alkali to the vitriolic ; and hence a decompofition might well 
take place, even on the fuppofition that the affinity of both 
acids to the alkali was equal : therefore, to attain any certainty 
in this matter, it is neceffary to afcertain the quantity and force 
of each of the attractive powers, and denote it by numbers. 
Mr. morveau was the fird who perceived the neceffity of 
th is calculation, and he has accordingly communicated to us, in 
numbers, a table of the attractive power of mercury with 
refpeCt to metals ;• but his method is incapable of being genera- 
5 lized. 
