Prior to the 19th Century

During this long period, humankind engaged in a global combinatorial experiment to explore the flora and fauna in the environment for their healing properties. This was the age of traditional medicines, which were handed down from generation to generation either by local folklore or codification in pharmacopoeia. Some of these concoctions may have bordered on witchcraft and were of dubious value. However, the majority would not have withstood the test of time if they were without merit. 

It is worth remembering that these advances are enjoyed only by a privileged few. The WHO estimates that 80% of the world s population still relies on traditional medicines for their primary health care needs. These are invariably crude natural product extracts, with potentially large variations in composition from one batch to another, and that are not subjected to controlled clinical trials. It is also true that many of these traditional medicines, although consumed by millions of people who vouch for their efficacy, have not succumbed to Western-style inquisitions to determine their mechanism of action. Traditional medicines, contend their proponents, work by complex synergistic interactions between their constituents. Consequently, reductionist approaches that identify single active pharmaceutical ingredients are doomed to failure. 

This phase saw continuous advances in chemistry and medicine. Organic chemistry was now capable of the synthesis of relatively complex targets by multistep sequences. The concept of systematically preparing a compound series and testing it for a biological effect became common, as exemplified in Ehrlich s quest for antisyphilitic agents. More and more pure natural products were identified and their structures elucidated. Synthetic mimics became a reahty. For example, amphetamines were inspired by the natnral prodnct lead, ephedrine. 

Quinine, plant (1820) antimalarial, disrupts polymerization of toxic haem breakdown products 

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Prior to the 19th century, practitioners of the healing arts knew essentially nothing about either the chemical composition of natural products or the mechanisms by which they work. They relied entirely on tradition, and trial and error, in the choices they made of the substances they used in their work. 

Nuclear dating has been most helpful in establishing the history of the earth and of the moon and of the meteorites. The fact is, there is no other way of measuring their ages. Prior to the discovery of natural radioactivity in the late 19th century, indirect methods were used to estimate the age of the earth, but there were no real answers until the radioactivity of thorium, uranium, and potassium were discovered and we began to understand atomic structure and to realize that nuclear transformation was essentially independent of the chemical form. 

The role of the early work on chemical kinetics in the evolution of physical chemistry has been examined with reference to van t Hoff s, Ostwald s, and Harcourt s researches prior to the 1880s.123 There is also a discussion of chemical kinetics and thermodynamics during the 19th century,124 and an analysis of the relation of chemical kinetics and physical chemistry up to the early part of the 20th century.125 Studies have been made of the role of instruments and the specific laboratory locales for chemical kinetics in the interwar years,126 and the work of H. Eyring127 and J.-A. Muller128 in chemical kinetics has been analysed. 

Most historians divide the Indian historical scene into several different ages or periods. These periods include the Pre-Vedic Age—all of the time prior to 1500 B.C., the Vedic Age from 1500-600 B.C., the Classic Period from 600-1200 A.D., the Medieval Age from 1200 A.D. to the end of the 18th century, the British Period through most of the 19th century, the Pre-Independence Period from 1900 to about 1947 and the Post-Independence Period from that time on. 

The history of American natural cement began in the early 19th Century canal-building era. Young s 1817 compilation. Internal Navigation, provides a snapshot of the state of technology in canal construction and operation at the time just prior to the commencement of construction of the Erie Canal and the discovery of natural cement rock in Fayetteville, New York. The excerpt details the earlier uses of Dutch trass in Ume-pozzolan mortars for canal construction, and predicts that the limestone materials required to produce hydraulic mortars wiU be found in New York State. 

The birth and adolescence of the synthetic dye industry in the late 19th and early 20th centuries prior to WWI has been a case study in international business and economics. Fundamental lessons about building competitive advantages and industry dynamics that led to German companies ascendance over British, French, European and American rivals have been analyzed meticulously32. 

Another famous appHcation of mechanistic chemistry was contributed by Morris Kharasch (1895-1957) at the University of Chicago. It was well known since the mid-19th century that addition reactions of HX (HCl, H-OH, etc) to unsymmetrical alkenes occur in a manner that places hydrogen on the olefinic carbon attached to more hydrogen atoms than the other olefinic carbon (Markovnikov s rule). However, when HX is hydrogen bromide (HBr), addition is typically anti-Markovnikov. Kharasch found that traces of peroxides (commonly present on glassware surfaces) initiate a free radical chain reaction for HBr (not for other HX). Careful removal of peroxides Ifom glassware prior to reaction removes the potential free radical mechanism and HBr adds in a normal Markovnikov mode (intermediacy of carbocation, rather than Ifee radical, intermediates). 

As greater interest in this monomer developed, new methods, such as the dehydration of phenyl ethyl alcohol (5) and the dehydrogenation of ethylbenzene (6),were used to obtain styrene monomer. However, in spite of considerable work by leading chemists of the 19th and 20th centuries, there was little information available on the polymerization of styrene prior to Staudinger s synthesis of polystyrene in the 1920 s (7). 

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