viii PREFACE.

pound, and depositing the earth which it had taken upbefore.

From this compound, fixt alcaline salts absorb the acid,and set at liberty the volatile alcali with all its originalproperties. Though the acid and fixt alcali, separately,are very pungent and corrosive, and so strongly disposed tounite with water that they imbibe it from the air, yet thecombination of the two has only a mild bitterish taste, anddissolves in water very difficultly and sparingly.

After all these transpositions, the acid may still be reco-vered pure, and made to pass again through the fame andthrough a multiplicity of other combinations. From al-most all its combinations it may be transferred to inflam-mable matter, and from the inflammable matter to iron :from the brimstone, which it forms with the one, the acidmay be obtained by burning with a proper apparatus j andfrom the vitriol, which it forms with the other, by distil-lation.

It is obvious, that in all these cases, the action is notbetween bodies considered as aggregates or masses, but be-tween the insensible and dissimilar parts of which they arecomposed j that the several effects can be regarded nootherwise than as simple facts, not reducible to any knownmechanism, not investigable from any principles, and eachdiscoverable by observation only ; and that the powers, onwhich they depend, are, so far as can be judged in thepresent state of knowledge, of a different kind from those,by which bodies tend to approach or cohere with forcesproportionate to their distances, or to resist or propel accord-ing