and best grows by a like periodicity in them.  This is our point—­that it is a peculiarity and law of mind, growing out of the very nature of mind and of its knowings, that no truth or knowledge which is in its nature a consequent on some other truths or knowledge, can by any possibility be in reality attained by any mind until after that mind has first secured and rightly appreciated those antecedent truths or knowings.  No later or more complex knowledge is ever comprehensible or acquirable, until after the elements of knowledge constituting or involved in it have first been definitely secured.  To suppose otherwise, is precisely like supposing a vigorously nourishing foliage and head of a tree with neither roots nor stem under it; it is to suppose a majestic river, that had neither sufficient springs nor tributaries.  Now, for the pupil, the text-book maker, the educator, no truth is more positive or profoundly important than this.  He who fails of it, by just so much as he does so, fails to educate.  Let the pupil, as he must, alternately study and not study—­go even on the same day from one study to a second, though seldom to more than a third or fourth.  By all this he need lose nothing; and he will tax and rest certain faculties in turn.  But then, insist that each subject shall recur frequently enough to perpetuate a healthy activity and growth of the faculties it exercises, usually, daily for five days in a week, or every other day at farthest; that each shall recur at a stated period, so that a habit of mind running its daily, steady and productive round with the sun may be formed; and that in and along the material of every subject pursued, whether it be arithmetic, or grammar, or chemistry, or an ancient or modern language, the mind shall so be enabled to advance consecutively, clearly and firmly from step to step—­from observation to law, from law to application, from analysis to broader generalization, and its application, and so on—­that every new step shall just have been prepared for by the conceptions, the mental susceptibility and fibre, gotten during the preceding ones, and that thus, every new step shall be one forward upon new and yet sure ground, a source of intellectual delight, and a further intellectual gain and triumph.  Need we say, this is the ideal?  Practice must fall somewhat short of it; but Practice must first aim at it; and as yet she has scarcely conceived about the thing, or begun to attempt it.  In truth, Practice is very busy, dashing on without a due amount of consideration, striving to project in young minds noble rivers of knowledge without their fountains; and building up therein grand trees of science, of which either the roots are wanting, or all parts come together too much in confusion.