PREFACE. vii I fhall alfo make fome alteration, and give a more full account of the fub- ftances which come under the head of rocks, or mineral fubflances, whofe com ponent parts are varioufly mixed, or blended together; as I have had lately fome, and fhall foon have greater oppor tunities, of exploring that fubject my- felf, with greater attention ; for which opportunities, I acknowledge with plea sure, that I am greatly indebted to a valuable Friend and a countryman of mine, whofe noble charaaer and bene volent deeds towards the relief of mis fortune and indigence, are greatly felt and acknowledged in his own country; while his laudable intentions, and inde fatigable ardour to collea, cultivate, and promote the ufeful fciences, will hardly efcape the attention of the great body of philofophical men in this country, whole