And yet consider the importance of such study. Changes around him appeal most strongly to the child. "Why _does_ this thing _do_ as it _does_?" is more frequent than "Why _is_ this thing as it _is_?" He sees changes of place, of form, of size, of composition, taking place; his curiosity is aroused; and he is ready to study with avidity, and in a systematic manner, the changes which his teacher may present to him. Consider the peculiarities belonging to the study of changes of any sort. The interest is held, for the mind is constantly gaining the new. The attention cannot be divided--all parts of the change, all phases of the action, must be known, and to be known must be _observed_; while in other forms of lessons the attention may be diverted for a moment to return to the consideration of exactly what was being observed before. It goes without saying that in one case quick and accurate observation, a retentive memory, and the association of causes and effects follow, and that in the other they do not. I advocate, therefore, the teaching of physical science in our schools--_in all our schools_. Physical science taught by the experimental method. An experiment has been defined as a question put to Nature, a question asked in _things_ rather than in _words_, and so conditioned that no uncertain answer can be given. Nature says that all matter gravitates, not in words, but in the swing of planets around the sun, and in the leap of the avalanche. And men have devised ingenious machines through which Nature may tell us the invariable laws of gravitation, and give some hint as to why it is true. There are two kinds of experiments, and two corresponding kinds of investigators. I. In original investigation there are the following elements: 1. The careful determination of all the conditions under which the experiment takes place. 2. The observation of exactly what happens, with a painstaking elimination of all previous notions as to what ought to happen. 3. The change of conditions, one at a time, with a comparison of the results obtained with the changes made, in order to determine that each condition has been given just its appropriate weight in the experiment. 4. The classification and explanation of the result. 5. The extension of the knowledge gained by turning it to investigations suggested by what has already been learned. 6. The practical application of the knowledge gained. II. In ordinary experiments for educational purposes the experimenter follows in a general way in the footsteps of the original investigator. There are the following elements to be considered: