Berthollet and the Soda Lakes
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Claude-Louis Berthollet, from Description de l’Égypte Antiquités, Mémoires v 2.

Claude-Louis Berthollet was already a very well-established scientist by the time Napoleon asked him to go on the Egyptian expedition. With Lavoisier, he is credited for having established the modern system of chemical nomenclature. He introduced the use of chlorine as a bleach, determined the composition of ammonia, and was elected to the French Academy of Sciences in 1780. Berthollet and the mathematician Gaspard Monge were the two leading scientists in the Commission of Sciences and Arts.

Berthollet was intrigued by the Natron Lakes that lie in a large depression west of Cairo. Natron is the Greek word for soda, or sodium carbonate, which can occur naturally in arid regions and has been mined from the dry lake bottoms in Egypt since ancient times for use in the preparation of mummies and the manufacture of glass. Visiting the Natron Lakes, Berthollet observed soda deposits on the surrounding limestone hills. In this natural laboratory, he reasoned, a chemical reaction occurred between salt (sodium chloride) and the limestone (calcium carbonate) in the hills to produce soda (sodium carbonate) and an accompanying product, calcium chloride, which seeped away into the ground. The reaction was the reverse of the one that chemists knew under laboratory conditions, and this indicated to Berthollet that physical conditions, such as heat and pressure, could affect the course of a chemical reaction.

Map of the Nile Delta, with the Natron Lakes at the left, from Description de l’Égypte Antiquités, Mémoires v 2.

Berthollet published his “Observations sur la natron” in the first volume of the Mémoires sur l’Égypte, and then went on in the next volume to give a more general treatment of the law of chemical affinities. He was planning to read this second paper to the Institute members in August 1799, but instead he was summoned to accompany Napoleon when he secretly fled back to Paris on August 22. However, his early return did allow him to complete his major book, Essai de statique chimique (Paris, 1803). The Essai is recognized as a landmark work that helped found the systematic study of physical chemistry.

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