It is also worthy of remark here, that the retention or permanence of the ideas thus committed to the keeping of the memory depends upon two circumstances. The first is, the vigour of the mental powers, or the intensity of the impression made upon them at the time of reiteration;--and the second, and certainly the principal circumstance, is the frequency of their reiteration by the mind. In evidence of the first we see, that a fall, a fright, or a narrow escape from imminent danger, although it occurred but once, and perhaps in early infancy, will be remembered through life; and in proof of the second, we find, that the scenes and circumstances of childhood being frequently and daily reiterated by the mind, at a time when it has little else to reiterate, remain permanently on the memory. The object therefore most to be desired by the teacher, is an exercise, or a series of exercises, by which, in his attempts to communicate knowledge to his pupil, this act of reiteration may be secured, and if possible repeated at pleasure, for more permanently fixing on the memory the knowledge communicated. In a former chapter we shewed, that this act of reiteration is the instrument employed by Nature for cultivating the powers of the mind as well as for communicating and impressing knowledge;--and we have also shewn that Nature in that process was successfully imitated by means of the catechetical exercise. This exercise has accordingly been found as powerful and efficient in promoting this, her second object, as it is in the first. The success of the catechetical exercise in communicating knowledge clearly to the young, even when it is but imperfectly managed, has been extensive and uniform; but wherever its nature has been properly understood, and it has been scientifically conducted, the amount of knowledge communicated in a given time, and with a given amount of mental and physical labour, stands confessedly without a parallel in the previous history of education. Minds the most obtuse, habits of listlessness the most inveterate, and mental imbecility, bordering on idiotcy, have been powerfully assailed and overcome; and knowledge, by means of this exercise, has forced its way, and firmly secured a place for itself, in minds which previously were little more than a blank. The causes of its success in cultivating the powers of the mind were formerly explained; but its adaptation to the communicating of knowledge is still more peculiarly striking. We shall endeavour to point out a few of these peculiarities.