MEASUREMENT OF THE ELASTIC FORCE OF GASES APPARATUS FOUNDED ON THE PROPERTIES OF AIR PRESSURE ON BODIES IN AIR. BALLOONS 1. Definition of physics.-The word physics is derived from the Greek, ovo, nature, for the ancients understood by the term physics the study of the whole of nature. They comprised within the domain of this science mechanics, astronomy, chemistry, botany, zoology, medicine, and even astrology and divination, whether by the stars or by the observation of physiognomy. The province of physics is at present much more restricted. Its object may be considered to be the study of those phenomena which do not depend on changes in the composition of bodies; these belong to chemistry. Thus, when water by cooling is changed into ice, and by heat this ice is again changed into water, the liquid is exactly the same as before; not merely are all its properties the same, but its substance is identical with what it originally was. The passage of water to the state of ice, and the return of the latter to the liquid state, are physical phenomena. In like manner, when a brittle object, of porcelain or glass, for instance, falls to the ground and breaks, each piece retains exactly the same chemical composition. The fall of the vessel and its fracture against the ground are then physical phe nomena. On the other hand, when wood burns, its substance is profoundly B |