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By 1590 Galileo Galilei had developed a number of criticisms of the Aristotelian system's view of the physical world. Primary among these was his theory on falling objects. In 1591, he demonstrated from the leaning tower of Pisa that weights of one pound and one hundred pounds, dropped from the top of the tower at the same time, hit the ground at the same time. Aristotle's claim that the rate of fall was determined by the weight of an object was thus overthrown, and replaced by Galileo's correct theory that the Earth's gravity produced a universal acceleration of objects toward its surface. Galileo is most important to the history of physics for his insistence upon viewing the world in terms of calculable forces and measurable bodies, and his experimental employment of this concept. In his 1638 Discourses Concerning Two New Sciences, Galileo explores the relative strength of various physical structures and his flawed theory on the attraction of separate particles which produced solid objects. He goes on to reject the Aristotelian explanation of the acceleration of falling bodies and substitutes his own, which has become the foundation of modern dynamics.
In the years after Galileo's work, much work was done in the pursuit of a more complete understanding of the character and conduct of matter. During the Middle Ages, alchemists, considered experts on matter, considered all matter to be made from four main elements: earth, air, water, and fire. They believed that the variety of matter they observed resulted from varying combinations of these four elements, and argued that if one could adjust the proportions of these elements one could translate one type of matter into another. Many alchemists spent their lives attempting to turn lead into gold. Very little of scientific worth emerged from this school of thought. One exception was the work of Jan Baptist Van Helmont of Belgium. He experimented on the role of water in the growth of plants, claiming that plants drew all of their substance from water. He also demonstrated that gases, though they commonly appeared similar, could be quite different in character. In fact, van Helmont invented the word 'gas.'
A large step in the understanding of the properties of gases was the invention of the barometer, to measure air pressure, by Evangelista Torricelli in 1643. In 1656 Otto von Guericke invented the air pump, and did the first experiments with vacuums, demonstrating many of the properties of gasses, such as the (until then) disputed claim that they did, in fact, have weight. Von Guericke also experimented a bit with electricity. Using von Guericke's air pump, Robert Boyle and Robert Hooke of Oxford examined the elasticity, compressability, and weight of the air. Boyle demonstrated later that only part of the air was used in respiration and combustion, an important finding that earned Boyle and Hooke credit as the discoverers of oxygen. Boyle's Law, widely applied in physics and chemistry, states that the volume of a gas varies inversely proportionally to the pressure exerted upon it.
Boyle also worked extensively with more purely chemical experiments, his book, The Skeptical Chymist (spelled with a 'y' in the original), debunks the Aristotelian view of the four elements and suggests the use of chemical indicators for the detection of acidic and basic liquids. In Boyle's Origin of Forms and Qualities, published in 1666, he assumes the existence of a universal type of matter, common to all bodies, and divisible into its smallest components, which correspond to what are known today as atoms. He described the structure of these smallest particles and the secondary structures that we know as molecules. Though his views were largely flawed, they contributed greatly to the study of the properties of matter.
The field of physics profited perhaps more than any other from the advances made in mathematics, as physical phenomena could now be explained through the quantification of forces that brought them about. Galileo was the first to insist upon the quantification of these forces, arguing that only if quantified would these forces lend themselves to logical description and understanding by the human mind. Galileo's work in defining the properties of motion paved the way for future physicists, and indeed, left the study of dynamics only a short step from its utmost extension under Isaac Newton, who extrapolated Galileo's theories into the laws of motion, and extended the Galileo's mathematical theories on the laws of gravity into fluxional calculus, which would become the cornerstone of modern physics.
Advances in physics constituted a sort of centerpiece in the evolution of scientific knowledge during the Scientific Revolution. They were made possible by advances in mathematics, which had linked pure numerical mathematics to geometry and subsequently linked the new geometry to motion. The advances in physics then gave birth to advances in astronomy, which applied the growing knowledge of physics to the entire universe rather than simply to terrestrial phenomena. However, during the immediate time of discovery, theories of physics were generally applied solely to earthly phenomena.
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