Pierre Louis Dulong found the relationship between the specific heat of an element and its atomic weight

12 February 1785, Rouen (France) - 19 July 1838, Paris (France)

He attended primary and secondary schools in Auxerre and Rouen, and at the age of 16, he entered the Ecole Polytechnique in Paris (France). There he was a student of two of the most brilliant chemists of his time: Claude Louis Berthollet and Louis Jacques Thénard.

After completing his higher studies, he qualified as a doctor and began to practise his profession. However, his future was not very promising in this field, as he devoted himself especially to offering his services to the needy in one of the poorest districts of Paris. He even opened an account in a pharmacy to provide medication to poor patients.

Dulong gave up medicine and began working in the chemistry laboratory owned by the aforementioned Berthollet in Arcueil. Later on, Thénard, noticing the young Dulong's great value, offered him a job as an assistant at the polytechnic school where he had studied.

What seemed to be a stroke of good luck in his fortuitous discovery of nitrogen trichloride (1813) turned into a small tragedy that cost him the sight of one eye and almost one hand. The accident involving this highly explosive and very delicate-to-handle compound did not deter him from his experiments on the subject.

Dulong's most important work in the field of physics was carried out jointly with Alexis-Thérèse Petit, professor of physics at the Ecole Polytechnique. In 1817 they both showed that Newton's law of refrigeration was true only for small temperature differences. Their work on temperature measurement and heat transfer (1818) was awarded a prize by the French Academy.

A year later, their experiments on expansion and measurement of temperatures and heat transfer and specific heat of gases led them to establish the empirical law on specific heats, known as Dulong and Petit's Law. This law would later be used in the determination of atomic weights.

In 1820, Dulong became Berthollet's assistant and later professor of physics at the École Polytechnique, of which he was appointed director ten years later (1830).

In 1826, he was elected a foreign member of the Royal Society.

In 1829 he found that, under the same conditions of pressure and temperature, equal volumes of all gases give off or absorb the same amount of heat when they are rapidly expanded or compressed to the same fraction of their initial volumes. He also deduced that the temperature changes accompanying these changes are inversely proportional to the heat capacities of the gases at constant volume.

With François Arago, he published a study of the elasticity of steam at high temperatures (1830). His last article (1838) describes the experiments he carried out to determine the heat developed in a chemical reaction.

In addition to the discovery of nitrogen trichloride (or azo chloride), the Law of Dulong and Petit, and his research on water and steam in collaboration with Arago, Dulong also succeeded in synthesising hypophosphoric acid, published a study on the density of various fluids, and measured the refractive index and the speed of sound in different gases.

Among his most important works are "On some combinations of azo with oxygen"; "On some combinations of phosphorus with oxygen"; "On the measurement of temperatures and the laws of heat communication"; and "On the elastic force of water vapour".

Pierre-Louis Dulong is one of the 72 scientists whose name is inscribed on the first floor of the Eiffel Tower. In addition, Dulong's name was given to a street in Paris on the right bank of the Seine.

Dulong died of stomach cancer in Paris on 19 July 1838. At the time of his death, he was working on the development of precise methods in calorimetry.