The volumes here presented to the reader have been, at intervals, for many years the occupation of the Author's leisure. The subject is one which can hardly fail to interest the mind desirous of investigating the History of Knowledge. Of that History it forms one chapter, — but a chapter the most poetical of all. The Author is well aware both of the difficulties which attend the topic, and of his own deficiencies; and had not the ground been almost unbroken, he would hardly have undertaken the task.