556 MAN IS MASTERING HIS MATERIAL WORLD tessence plus hotness and dryness. for nearly eighteen centuries. Aristotle's teachings were accepted The Alchemists Learned Many Things about the Chemical Properties of Matter. About three centuries before Christ, Alexandria began to replace Athens as the intellectual center of the ancient world. The Egyptians had long practiced the art of preparing cheap alloys which resembled silver and gold. Under Aristotle's influence, the idea gradually developed that, with the proper essence, base metals such as lead could actually be changed into gold. Many people began experimenting to find this essence. It was, of course, never discovered, but some experimenters actually succeeding in preparing materials that resembled gold and probably believed that they had discovered the important essence. Many other experimenters were outright fakers. When one considers how the "patent medicines" industry flourishes today, he can understand why these alchemists were able to exist and even secure the support of kings and rulers in those dark ages of ignorance and .superstition. It should be noted, however, that a tremendous amount of knowledge accumulated as a result of these experiments. In the seventh century a.d. the Arabs conquered Egypt, and alchemy became an Arabian art. One of the outstanding Arabian alchemists, Geber (721-813), believed that all metals were composed of sulfur and mercury. His influence was still felt as late as 1500 a.d., when Paracelsus (1493-1541) held that the three elements — earth, air, and water — were represented by salt, sulfur, and mercury, respectively. Paracelsus was the first of the iatrochemists, that is, the chemists who devoted their lives to the search for medicines to cure disease. From his time, the alchemists came to be recognized as rogues, and alchemy acquired a bad reputation. At the same time, the iatrochemists worked earnestly and honestly to obtain new substances that would benefit humanity. Paracelsus described these workers as follows: They are not given to idleness, or go in a proud habit, or plush and velvet garments, often showing their rings on their fingers, or wearing swords with silver hilts by their sides, or fine and gay gloves on their hands but diligently follow their labors, sweating whole days and nights by their furnaces. They do not spend their time abroad for recreation but take delight in their laboratories. They put their fingers among coals, into clay and filth, not into gold rings. They are sooty and black, like smiths and miners, and do not pride themselves upon clean and beautiful faces. The history of alchemy and the iatrochemical period is of great interest, but we must now turn to the early discoveries concerning the nature of matter made by the pioneers of modern chemistry.
: THE MATERIALS OF THE UNIVERSE 557 Robert Boyle First Distinguished between Mixtures, Compounds, and Elements. Robert Boyle (1627-1691) early became a member of the College, which consisted of a group of men who met together in Invisible England from time to time to discuss philosophical and scientific questions. They adopted the experimental and inductive method advocated by Francis Bacon, "being satisfied that there was no certain way of arriving at any competent knowledge unless they made a variety of experiments upon natural bodies." This group of learned men was similar to other groups which sprang up about the same time in Germany, France, and Italy. Later this group developed into the Royal Society, one of the world's greatest scientific societies of today. In 1661 Boyle published his book. The Sceptical Chymist, in which he distinguished for the first time between mixtures, compounds, and elements I mean by elements certain primitive and simple, or perfectly unmingled bodies; which not being made of any other bodies, or of one another, are the ingredients of which all those called perfectly mixt bodies are immediately compounded, and into which they are ultimately resolved. Chemical Changes Are Changes in the Composition of Matter. Robert Boyle introduced the term analysis, which refers to the process of determining the composition of a given sample of material. Analysis is simply a method of separating materials so that their individual properties may be observed. Some materials are obviously heterogeneous in nature; different particles of material exhibit different properties. Granite is an example of a heterogeneous substance; crystals of feldspar are obviously different from the quartz in which they are embedded. A mixture of sand, sugar, and iron filings is obviously heterogeneous, for the white sugar particles can be separated by dissolving the sugar in water and can be recovered by evaporation of the water; and the iron filings may be removed with a magnet. All such heterogeneous materials are called mixtures; the test of a mixture is that its ingredients can be separated by physical means, that is, by methods which do not change the composition of any of the components of the mixture. The sugar, on the other hand, is said to be homogeneous, because every particle of it exhibits the same properties. Iron and sand are likewise recognized as homogeneous materials. Any pure (homogeneous) material of invariable composition is called a substance. Mixtures are therefore composed of two or more substances which have been intermingled in such a way that the substances have not been