_Porosity_ is another attribute of Matter, for in all Matter there are pores, or spaces, between the particles. Sometimes such openings are plainly visible; in very “solid” bodies they are, to a great extent, indistinguishable. But we know that the spaces exist, because we can _compress_ the particles together. _Inertia_ is also a general property of Matter, and the meaning of the term is “inactivity,” or passiveness—a want of power in an object to move, or when moving, to stop of itself. It will come to rest apparently by itself, but the resistance of the air and the friction of the ground, or the attraction of the earth, will really occasion the stoppage of the object. We will speak more fully of Inertia presently. Elasticity and Expansibility are evident in fluids and gases. We have thus introduced our readers to some of the most evident facts connected with Matter. The various Forces and Phenomena of attraction will be fully considered farther on; at present we are about to show our readers how they may first profitably study Science in the open air for themselves, and we will give them our experience of the Book of Nature. CHAPTER II. SCIENCE IN THE OPEN AIR—APHIDES—EVAPORATION BY LEAVES—AN AQUARIUM—THE CATALEPTIC FOWL—NEEDLE POINTS AND THORNS—MICROSCOPIC AQUARIUM—CAPE GRISNEZ—CRYSTALS—ICE ON THE GAS LAMPS. [Illustration: Fig. 1.—Ants engaged in extracting aphides from a rose-tree (highly magnified)] Some years ago we were staying in Normandy, not far from the town of C——, enjoying, in the midst of most cordial hospitality, the peacefulness of country life; and my kind hosts, with me, took great pleasure in having what we called “a course of science in the open air.” The recollections of that time are some of the pleasantest in the whole course of my life, because all our leisure was intelligently occupied. Each of us set himself to provide the subject of some curious observation or instructive experiment; one made a collection of insects, another studied botany. In the daytime we might have been seen examining, under a magnifying glass, the branch of a rose-tree, from which the ants were endeavouring to extract the aphides[1] (fig. 1). At night we admired through the telescope the stars and planets that were visible; or if the sky was not clear, we examined under a strong magnifier grains of pollen from flowers, or the _infusoria_ in a drop of stagnant water. Frequently some very insignificant object became the occasion for some scientific discussion, which terminated with an experimental verification.