A system of chemical substances at equilibrium, even though having an unchanging composition, is most often not static; molecules of the substances continue to react with one another thus giving rise to a dynamic equilibrium. The discoveries of the chemical elements has a long history culminating in the creation of the periodic table by Dmitri Mendeleev. A related concept free energy, which also incorporates entropy considerations, is a very useful means for predicting the feasibility of a reaction and determining the state of equilibrium of a chemical reaction, in chemical thermodynamics. Fields of specialization include biochemistry, nuclear chemistry, organic chemistry, inorganic chemistry, polymer chemistry, analytical chemistry, physical chemistry, theoretical chemistry, quantum chemistry, environmental chemistry, and thermochemistry. Chemists carefully describe the properties they study in terms of quantities, with detail on the level of molecules and their component atoms. British William Prout first proposed ordering all the elements by their atomic weight as all atoms had a weight that was an exact multiple of the atomic weight of hydrogen. In comparison, chemists who have obtained a Master of Science (M.S.) [48] Alchemy continued to be developed and practised throughout the Arab world after the Muslim conquests,[49] and from there, and from the Byzantine remnants,[50] diffused into medieval and Renaissance Europe through Latin translations. Chemistry typically is divided into several major sub-disciplines. Postdoctoral experience may be required for certain positions. Many physical chemists specialize in exploring and proposing the mechanisms of various chemical reactions. Because of these factors affecting exact job titles with distinct responsibilities, some chemists might begin doing technician tasks while other chemists might begin doing more complicated tasks than those of a technician, such as tasks that also involve formal applied research, management, or supervision included within the responsibilities of that same job title. A phase is a set of states of a chemical system that have similar bulk structural properties, over a range of conditions, such as pressure or temperature. [34] In 1663, the chemist Christopher Glaser described "chymistry" as a scientific art, by which one learns to dissolve bodies, and draw from them the different substances on their composition, and how to unite them again, and exalt them to a higher perfection. They can be analyzed using the tools of chemical analysis, e.g. Under the influence of the new empirical methods propounded by Sir Francis Bacon and others, a group of chemists at Oxford, Robert Boyle, Robert Hooke and John Mayow began to reshape the old alchemical traditions into a scientific discipline.