THE substance of the following pages was delivered, in the form of Lectures, to the members of the Philosophical Institution of Edinburgh, in the months of March and April, 1858. Passages necessarily omitted in the delivery, are here restored; a few passages spoken from notes, are here expanded from recollection; and there are also some additions, especially towards the end. By these changes the Discourses are made to exceed by much the ordinary limits of Lectures. I have, however, retained the name
of "Lectures" by way of title, - partly because