It may appear to some, that this truth is so plain and obvious, as to require no further illustration or enforcement.--We sincerely wish that it were so. But long experience justifies us in being sceptical on the point. And as the establishment of this principle, and a thorough knowledge of its working, are perhaps of more value than any other truth in the whole range of educational science, we shall offer a few remarks on its validity and importance, before proceeding to examine the means by which Nature carries it into operation. That knowledge, when once acquired, is intended by Nature to be put to use, is proved negatively by the well known fact, that almost all our _mental_ acquirements, when not used, are soon lost. They gradually fade from the mind, and are at last blotted from the memory. Hence the disappearance in after life of all the academical and collegiate acquirements of those youths who move in a sphere where their use is not required; and of those portions of the early attainments of even professional men, which are not necessary for their particular pursuits. By the universal operation of this principle, Nature gives fair warning of the folly of useless learning; and plainly indicates, that whenever the benefits which she confers are not put to use as she designed, they will gradually, but most certainly, be withdrawn. The same fact is also proved positively:--For we find, that the proper use of any portion of our knowledge, is invariably rewarded by its becoming still more familiar. The student who puts a principle in chemistry to the test of experiment, will understand it better, remember it longer, and be able to apply it to useful purposes, much more readily than his companion who merely reflects upon it. And of two individuals, who by a lecture have been taught the duty and the delights of mercy, that one will learn it best, and remember it longest, who, immediately on hearing it, is prompted to relieve a fellow creature from distress, or to save a family from ruin.