---
title: "History of chemistry"
chunk: 10/20
source: "https://en.wikipedia.org/wiki/History_of_chemistry"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T03:59:28.102404+00:00"
instance: "kb-cron"
---

=== Mid-1800s ===
In 1840, Germain Hess proposed Hess's law, an early statement of the law of conservation of energy, which establishes that energy changes in a chemical process depend only on the states of the starting and product materials and not on the specific pathway taken between the two states. In 1847, Hermann Kolbe obtained acetic acid from completely inorganic sources, further disproving vitalism. In 1848, William Thomson, 1st Baron Kelvin (commonly known as Lord Kelvin) established the concept of absolute zero, the temperature at which all molecular motion ceases. In 1849, Louis Pasteur discovered that the racemic form of tartaric acid is a mixture of the levorotatory and dextrotatory forms, thus clarifying the nature of optical rotation and advancing the field of stereochemistry. In 1852, August Beer proposed Beer's law, which explains the relationship between the composition of a mixture and the amount of light it will absorb. Based partly on earlier work by Pierre Bouguer and Johann Heinrich Lambert, it established the analytical technique known as spectrophotometry. In 1855, Benjamin Silliman, Jr. pioneered methods of petroleum cracking, which made the entire modern petrochemical industry possible.

Avogadro's hypothesis began to gain broad appeal among chemists only after his compatriot and fellow scientist Stanislao Cannizzaro demonstrated its value in 1858, two years after Avogadro's death. Cannizzaro's chemical interests had originally centered on natural products and on reactions of aromatic compounds; in 1853 he discovered that when benzaldehyde is treated with concentrated base, both benzoic acid and benzyl alcohol are produced—a phenomenon known today as the Cannizzaro reaction. In his 1858 pamphlet, Cannizzaro showed that a complete return to the ideas of Avogadro could be used to construct a consistent and robust theoretical structure that fit nearly all of the available empirical evidence. For instance, he pointed to evidence that suggested that not all elementary gases consist of two atoms per molecule—some were monatomic, most were diatomic, and a few were even more complex.
Another point of contention had been the formulas for compounds of the alkali metals (such as sodium) and the alkaline earth metals (such as calcium), which, in view of their striking chemical analogies, most chemists had wanted to assign to the same formula type. Cannizzaro argued that placing these metals in different categories had the beneficial result of eliminating certain anomalies when using their physical properties to deduce atomic weights. Unfortunately, Cannizzaro's pamphlet was published initially only in Italian and had little immediate impact. The real breakthrough came with an international chemical congress held in the German town of Karlsruhe in September 1860, at which most of the leading European chemists were present. The Karlsruhe Congress had been arranged by Kekulé, Wurtz, and a few others who shared Cannizzaro's sense of the direction chemistry should go. Speaking in French (as everyone there did), Cannizzaro's eloquence and logic made an indelible impression on the assembled body. Moreover, his friend Angelo Pavesi distributed Cannizzaro's pamphlet to attendees at the end of the meeting; more than one chemist later wrote of the decisive impression the reading of this document provided. For instance, Lothar Meyer later wrote that on reading Cannizzaro's paper, "The scales seemed to fall from my eyes." Cannizzaro thus played a crucial role in winning the battle for reform. The system advocated by him, and soon thereafter adopted by most leading chemists, is substantially identical to what is still used today.

=== Perkin, Crookes, and Nobel ===
In 1856, Sir William Henry Perkin, age 18, given a challenge by his professor, August Wilhelm von Hofmann, sought to synthesize quinine, the anti-malaria drug, from coal tar. In one attempt, Perkin oxidized aniline using potassium dichromate, whose toluidine impurities reacted with the aniline and yielded a black solid—suggesting a "failed" organic synthesis. Cleaning the flask with alcohol, Perkin noticed purple portions of the solution: a byproduct of the attempt was the first synthetic dye, known as mauveine or Perkin's mauve. Perkin's discovery is the foundation of the dye synthesis industry, one of the earliest successful chemical industries.
German chemist August Kekulé von Stradonitz's most important single contribution was his structural theory of organic composition, outlined in two articles published in 1857 and 1858 and treated in great detail in the pages of his extraordinarily popular Lehrbuch der organischen Chemie ("Textbook of Organic Chemistry"), the first installment of which appeared in 1859 and gradually extended to four volumes. Kekulé argued that tetravalent carbon atoms – that is, carbon forming exactly four chemical bonds – could link together to form what he called a "carbon chain" or a "carbon skeleton," to which other atoms with other valences (such as hydrogen, oxygen, nitrogen, and chlorine) could join. He was convinced that it was possible for the chemist to specify this detailed molecular architecture for at least the simpler organic compounds known in his day. Kekulé was not the only chemist to make such claims in this era. The Scottish chemist Archibald Scott Couper published a substantially similar theory nearly simultaneously, and the Russian chemist Aleksandr Butlerov did much to clarify and expand structure theory. However, it was predominantly Kekulé's ideas that prevailed in the chemical community.