Joseph-Louis Proust

Joseph-Louis Proust, also known as Luis Proust, (born Sept. 26, 1754, Angers, France—died July 5, 1826, Angers), French chemist who proved that the relative quantities of any given pure chemical compound’s constituent elements remain invariant, regardless of the compound’s source. This is known as Proust’s law, or the law of definite proportions (1793), and it is the fundamental principle of analytical chemistry. Proust also carried out important applied research in metallurgy, explosives, and nutritional chemistry.

Education and life

The son of an apothecary, Proust prepared for the same occupation, first with his father in Angers and then in Paris, where he also studied chemistry with Hilaire-Martin Rouelle.

In 1776 Proust was appointed a pharmacist at the Salpêtrière Hospital in Paris.

in 1778 Proust abandoned pharmacy to take a professorship of chemistry at the recently established Seminario Patriótico Vascongado in Vergara, Spain.

In 1780 Proust returned to Paris, where he taught chemistry at the Musée, a private teaching institution founded by scientific impresario Jean-François Pilâtre de Rozier. Part of this association involved Proust with aerostatic experiments, which culminated in a balloon ascent with Pilâtre on June 23, 1784, at Versailles, in the presence of the royal court.

In 1786 Proust returned to Spain to teach chemistry, first at Madrid and then in 1788 at the Royal Artillery School in Segovia. The school had been founded in 1764 as part of the program of the government of Charles III to bring Spain abreast of the northern European countries regarding military training. Proust’s chair (and an associated school of chemistry and metallurgy) had been proposed in 1784 to introduce artillery cadets to the latest relevant scientific training. Because of Spain’s scientific backwardness, expert instructors had to be sought abroad. Proust was recommended by no less than the great French chemist Antoine-Laurent Lavoisier.

Proust did not actually assume his chair until 1792, owing to a combination of bureaucratic inefficiency and his own exacting demands for laboratory equipment. When finally ready, his laboratory was undeniably one of the finest in Europe, and Proust probably did the bulk of his practical and analytical chemistry there. Difficulties with the military authorities, though, resulted in Proust’s transfer in 1799 to a chair in chemistry in Madrid.

In 1798 Proust married Anne Rose Chatelain Daubigne, a French resident of Segovia. They returned to France in 1806 under obscure circumstances and settled in Craon, near Angers. Upon the death of his wife in 1817, Proust moved to Angers, where he took over in 1820 the pharmacy of his ailing brother Joachim. Although Proust had returned to France in reduced circumstances, his scientific stature was recognized. He was elected to the French Academy of Sciences to succeed Louis Bernard Guyton de Morveau in 1816; he was made a chevalier of the Legion of Honour in 1819; and he was granted a pension by Louis XVIII in 1820.

Joseph Proust was best known for his analytical abilities. His experiments with inorganic binary compounds - mostly sulfates, sulfides, and metallic oxides - led him to formulate the Law of Constant Composition. The law was first published in a paper on iron oxides in 1794.

The Law of Constant Composition, discovered by Joseph Proust, is also known as the Law of Definite Proportions. It is different from the Law of Multiple Proportions although both stem from Lavoisier's Law of Conservation of Mass.

The French chemist Joseph Proust stated this law the following way: "A chemical compound always contains the same elements combined together in the same proportion by mass."

For example, pure water obtained from different sources such as a river, a well, a spring, the sea, etc., always contains hydrogen and oxygen together in the ratio of 1:8 by mass. Similarly, carbon dioxide (CO2) can be obtained by different methods such as,

• Burning of carbon
• Heating of lime stone
• Applying dilute HCl to marble pieces

Each sample of CO2 contains carbon and oxygen in a 3:8 ratio.

Joseph Proust made his discovery on constant composition in 1797 but it was not accepted by the scientific community until the year of 1808.  The discovery was made when Proust conducted several experiments that proved that cupric, also known as copper 2, always has 5.3 copper parts to 1 part carbon and to 4 parts oxygen.  Using this evidence, Proust was able to faithfully say that chemical compounds always have the same ration of elements.

When Dalton proposed his atomic theory, Proust’s law helped to confirm the hypothesis. According to Dalton, atoms would always combine in simple whole number ratios. For example, all water molecules are alike, consisting of two atoms of hydrogen and one atom of oxygen. Therefore, all water has the same composition. Proust’s law has been confirmed by experiments. For example, water always contains 11.2 percent hydrogen and 88.8 percent oxygen.