234 PHYSICAL LIMITATIONS HAVE BEEN OVERCOME Robert Boyle (1627-1691), famous for the law known by his name, improved the air pump, observed the effect of atmospheric pressure on the boiling-point of water, improved the thermometer by sealing it, and noted the unvarying temperature of the healthy human body. Robert Boyle believed heat to be the result of a brisk vibratory agitation of small particles of matter. Sir Isaac Newton (1642-1727) and Henry Cavendish (1731-1810) held this same view, but the knowledge of energy was too inadequate at that time for the proper development of their ideas. The contemporaries of Newton and Cavendish generally considered heat to be a subtle, invisible, weightless fluid. Even these great men could not furnish much evidence to refute this conception. The fluid theory of heat, like many other theories since rejected, proved its value in leading to greater knowledge. Joseph Black (1728- 1799), working on the basis of the fluid theory of heat, discovered and investigated the heat changes involved in the freezing and the boiling of water and assumed that the caloric (thermal) fluid united with ice to form water and with water to form steam. He originated the idea of specific heats and established the methods of measuring heats by calorimetry. It remained for Ilelmholtz and Joule, between 1840 and 1850, to demonstrate the mechanical equivalent of heat and thus prove that heat was a form of energy. Joule (1818-1889) forced water through narrow tubes, compressed masses of air, and heated liquids by the rotation of paddle wheels and found that, regardless of how the work was done, the same amount of work always developed the same amount of heat. Then it was that the concept of energy and the conservation of energy was born. Careful experiments led to the conclusion that energy and matter were separate entities and that energy could neither be created nor destroyed. It was only logical to reach the conclusion that matter and energy were eternal and indestructible, and it was only in recent years that these ideas had to be modified. The first adequate kinetic-molecular theory was published in 1857 by Clausius. It was improved in 1860 by Maxwell. The Kinetic-molecular Theory in Question-and-Answer Form. 1. Is matter continuous? Answer: No. If any portion of pure material be divided and subdivided, there will eventually be obtained the smallest unit of that substance which can continue to exist free (i.e., uncombined) in nature. This is called the molecule. A molecule can be divided into smaller particles but not without a change in its chemical properties.
THE KINETIC- MOLECULAR THEORY 235 2. What is the size of a molecule? Answer: Molecules are extremely small — so small, indeed, that they cannot be seen by the most powerful microscope. A row of 40,000,000 water molecules would measure an inch. If a liter of water were poured into the ocean and each water molecule were labeled in such a way that it could be identified a few million years later, after all the water of the earth was thoroughly and uniformly mixed, any liter of water then taken would be found to contain 2220 of the original molecules. It has been calculated that if the molecules of one cubic inch of air were each changed to a grain of sand, the resulting sand would fill a trench a mile wide and three feet deep reaching from New York to San Francisco. The British physicist, Aston, gives the following illustration: "If you made a hole in an ordinary evacuated electric light bulb which would allow the molecules of air to pass in at a rate of one million per second, it would be about a hundred million years before the bulb would be filled." 3. How far apart are the molecules? Answer: The molecules of gases are very far apart in relation to their size, inasmuch as the actual space occupied by the molecules of a given volume of gas is only a very small portion of this volume. 4. Are all molecules the same size? Answer: No. The molecules of a given substance have the same average size, but molecules of different substances have different average sizes. Hydrogen molecules, as compared with certain very large protein molecules, would be in the ratio of 1 to 33,000. 5. Do molecules move? Answer: Yes, molecules of gases at ordinary temperatures move very rapidly. The average speed of hydrogen molecules at room temperature is about a mile per second. 6. Do all molecules of a given substance under the same conditions move with the same speed? Answer: No. Most of the molecules move with speeds very near the average speed, but a few move with speeds very much more or very much less than the average speed. Today, according to the theory of probability, the many well-known laws, such as the gas laws, are expressions of probable tendencies, inasmuch as they deal with statistical averages rather than with individual particles. Thus it can be predicted within narrow limits how many babies will die in the United States in a year. But we cannot foretell how long one particular baby will live. The theory of probability, formulated by Pascal and Fermat in the seventeenth century, has been used to estimate the errors in physical measurement and to rationalize insurance and statistics. The Belgian astronomer Quetelet (1796-1874) showed that the chest measurements of Scottish soldiers gave a curve of variation that is