This, it is obvious, cannot be done by a loose, partial, and illarranged presentation of them, but only by a presentation that is strictly logical. Commonly, this foundation-work, as it may be termed, has to be done over and over again, bringing much unnecessary labor to both teacher and scholar, simply because it is not done right at the outset.

There is another consideration bearing upon this point, which is of great importance. It is essential to the successful study of science that good habits of mind be formed, and the earlier they are formed the better. I need not stop to show that clear logical presentations of facts and principles tend to form such habits, and that a loose, confused mode of presenting them tends to form habits of an opposite character.

Let me not be understood to advocate that prominence of logical framework, as it may be called, which is so common in books for instruction. With all this show of logical arrangement, there is often much that is really very illogical. With the beginner, at least, the less there is of the formalities of arrangement the better. And yet there should in reality be a strict regard to the proper logical order in introducing facts and principles to the mind of the learner. If this natural order be observed, every page that the student learns serves to prepare his mind for what comes after. There is no point in which books for instruction so often fail as in this.

Most books for the instruction of beginners in science, present a strange mixture of child's talk, and language that the child cannot understand, but can only learn by rote. Even the hard technical terms of science often enter abundantly into the compound. It seems to be forgotten that great simplicity of language may be the vehicle of even a deep philosophy, and is consistent with an elevated style. Clear, precise statement, logical order of arrangement, and felicitous illustration, are the elements of such a style. And these elements cannot exist, unless there be an appreciation in the writer's mind of the attitude of the minds that he addresses. He must not only see clearly the facts and principles of science himself, but he must know how to make others see them clearly also.

It is obvious that in a "First Book" there must be a less number of points introduced than in a book intended for instruction afterward. The field of vision should be gradually enlarged as the learner advances. At first there must be a selection of such points as he can most readily understand, reserving the more difficult ones for another book. And while the second book should be much more complete than that which is designed for beginners, yet in the latter the really essential and fundamental parts of the science should be clearly presented.

The principles thus advanced I have endeavored to follow in the construction of this little work. It is intended principally for the use of common schools, and yet, like my larger work on Physiology, it is adapted for general reading. It will prepare the reader and the scholar for the more full examination of the subject in the larger work.

I need hardly say, that in order to teach from this book satisfactorily, it is necessary for the teacher to read both books. By doing so, he will see clearly in every case the reason of the selection that I have made in this work from the facts that are presented so fully in the other, and will therefore be better prepared to teach according to the plan that I have in view. The questions that I have placed at the bottom of each page can be altered as the teacher thinks best, to suit the different capacities of his scholars. For certain general directions in teaching Physiology, I refer him to the Appendix of my larger work.