Joseph Louis Gay-Lussac (born:  December 6, 1778 / died: 1850)
A French chemist and physicist, known for his studies on the physical properties of gases. He was born in Saint Léonard and educated at the École Polytechnique and the École des Ponts et Chaussées in Paris. After holding several professorships he became professor of physics at the Sorbonne from 1808 to 1832.

In 1804 he made balloon ascensions to study magnetic forces and to observe the composition and temperature of the air at different altitudes. In 1809 he formulated a law of gases that is still associated with his name. Gay-Lussac's law of combining volumes states that the volumes of the gases involved in a chemical reaction (both reactants and products) are in the ratio of small whole numbers. In connection with these studies he investigated, with German naturalist Baron Alexander von Humboldt, the composition of water and found it forms when two parts of hydrogen and one of oxygen unite.

In 1809 Gay-Lussac worked on the preparation of potassium and boron and investigated the properties of chlorine and hydrocyanic acid. In the field of industrial chemistry, he developed improvements in various manufacturing and assaying processes. In 1831 he was elected to the Chamber of Deputies and in 1839 to the Senate.
The French Revolution affected many of what were to become the French scientific elite. Gay-Lussac was sent to Paris at the age of fourteen when his father was arrested. After having had private lessons and attending a boarding school, the Ecole Polytechnique and the civil engineering school, Gay-Lussac became an assistant to Berthollet who was himself a co-worker of Lavoisier. Gay-Lussac thus got the chance to become part of the group of famous men who spent time at Berthollet's country house near Arcueil. Here among the Arcueil Society he received his training in chemical research.
With the encouragement of Berthollet and LaPlace, Gay-Lussac at the age of 24 conducted his first major research in the winter of 1801-1802. He settled some conflicting evidence about the expansion properties of different gases. By excluding water vapor from the apparatus and by making sure that the gases themselves were free of moisture, he obtained results that were more accurate than had been obtained previously by others. He concluded that equal volumes of all gases expand equally with the same increase in temperature. While Jacques Charles discovered this volume-temperature relationship fifteen years earlier, he had not published it. Unlike Gay-Lussac, Charles did not measure the coefficient of expansion. Also, because of the presence of water in the apparatus and the gases themselves, Charles obtained results that indicated unequal expansion for the gases that were water soluble.
Gay-Lussac, like his mentor Berthollet, was interested in how chemical reactions take place. Working with the mathematical physicist, LaPlace, Gay-Lussac made quantitative measurements on capillary action. The goal was to support LaPlace 's belief in his Newtonian theory of chemical affinity. In 1814 this theoretical bent continued as Gay-Lussac and LaPlace sought to determine if chemistry could be reduced to applied mathematics. The approach was to ask whether the conditions of chemical reactions could be reduced simply to, as LaPlace had suggested, considerations of heat. 
As with his mentor before him, Gay-Lussac was consulted by industry and supported by the government. "Napoleonic science sharpened the appetites of young men by holding up the prospects of recognition and reward". Gay-Lussac and Thenard, the laboratory boy turned professor, isolated the element boron nine days before Davy's group did (but Davy was the first to publish. Gay-Lussac led his group into the isolation of plant alkaloids for potential medical use and he was instrumental in developing the industrial production of oxalic acid from the fusion of sawdust with alkali. His most important contribution to industry was, in 1827, the refinement of the lead chamber process for the production of sulfuric acid, the industrial chemical produced in largest volume in the world. The tall absorption towers were known as Gay-Lussac Towers. The process is: 
SO2 (g) + NO2 (g) -------> SO3 (g) + NO (g)
This reaction was carried out in a lead-lined chamber in which the sulfur trioxide was then dissolved in water to produce sulfuric acid. Gay-Lussac's contribution was a process for recycling the nitrogen monoxide after oxidizing it to NO2. Sulfuric acid was produced this way well into the twentieth century, when it was replaced by catalytic oxidation of SO2 in the "Contact Process".
While Gay-Lussac was a great theoretical scientist, he was also respected by his colleagues for his careful, elegant, experimental work. Wanting to know why and how something happened was important to Gay-Lussac, but he preferred knowing much about a limited subject rather than proposing broad new theories which might be wrong . He devised many new types of apparatus such as the portable barometer, an improved pipette and burette and, when working at the Mint, a new apparatus for quickly and accurately estimating the purity of silver which was the only legal measure in France until 1881. His work on iodine is considered a model of chemical research. His precise measurement of the thermal expansion of gases mentioned above was used by Lord Kelvin in the development of the absolute temperature scale and Third Law of Thermodynamics and by Clausius in the development of the Second Law. He and Thenard improved existing methods of elemental analysis and developed volumetric procedures for measuring acids and alkalis. His quantification of the effect of light on the reaction of chlorine with hydrogen elevated photochemistry from mere artifice into a theoretical science which culminated, fifty years after his death, in the quantum theory. An example of his dedication to meticulous experimenting is his decision to undertake a balloon flight to a record setting height of 23,000 feet to test an hypotheses on earth's magnetic field and the composition of the air.
The work for which Gay-Lussac is most remembered in high school and university courses of general chemistry is his Law of Combining Volumes: "The compounds of gaseous substances with each other are always formed in very simple ratios by volume". If we follow the development of this law we can see the scientific method at work, in all its beauty and nobility, and with its pitfalls, resting as it does on the frailty of human nature.

Gay-Lussac began with a statement of intent: "I hope by this means to give proof of an idea advanced by several distinguished chemists--that we are perhaps not far removed from the time when we shall be able to submit the bulk of chemical formula to calculation".

The events that culminated in the presentation of his memoir at Arcueil began with his balloon flight and measurements of the composition of the air. These studies led him to criticize a man ten years his elder--the scientist-explorer Alexander von Humbolt, who had also published measurements on the composition of the air. But in an illustration of the nobility of science, Humbolt, far from becoming angry with Gay-Lussac, saw that he had something to learn about precision in scientific research. The two became collaborators and friends and, in fact, eventually traveled together throughout Europe for a year in 1805, going to Rome, Switzerland and Berlin(3). Before that trip they worked on finding the ratio in which hydrogen and oxygen combine to form water. They needed this fact in order to find the percent of oxygen in the air. They came up with the remarkably accurate results of a volume ratio of 100 of oxygen to 200 of hydrogen.
What is surprising is that four years passed before Gay-Lussac published his now famous results. In the interim, during their trip together, he worked with Humbolt on measuring the earth's magnetic intensity. In 1807 he worked on a series of experiments to find out if there is a general relationship between the specific heats of gases and their densities.

Gay-Lussac looked at some previous data collected by Davy. This consisted of analysis of the proportions by weight of elements in three different oxides of nitrogen, as follows:

 Proportions by Weight

Nitrogen  Oxygen
Nitrous oxide 63.30  36.70
Nitrous gas 44.05 55.95
Nitric Acid 29.50 70.50
Next Gay-Lussac "reduces these proportions to volumes", using the specific gravities of the gases relative to hydrogen:

Proportions by Volume

Nitrogen Oxygen
Nitrous oxide 100 49.5
Nitrous gas 100 108.9
Nitric acid 100 204.7
Now he can accept (by what today we call the 5% rule) that "the first and last of these proportions differ only slightly from 100:50 and 100:200", but he recognizes that the second is significantly different from 100:100. He also knew that Berthollet had obtained the proportions by volume of H and N in ammonia, while Gay-Lussac, himself, had obtained those of sulfurous gas and oxygen in sulfuric acid and that these are indeed simple whole numbers. So he does an analytical experiment. He reacts 100 volumes of nitrous gas with "the new combustible substance from potash" (presumably potassium) and he finds that the volume of the gas decreases by 50% and the remnant is all nitrogen. The reaction presumably is:

                               4K (s) + 2NO (g) -------> 2K2O + N2

He can now write:

Proportions by Volume

Nitrogen Oxygen
Nitrous oxide 100 50
Nitrous gas 100 100
Nitric acid 100 200
In modern terms these three compounds are N2O, NO, and NO2. So Gay-Lussac has used the scientific method: Question, Hypothesis, Experiment (including careful measurement, reproducibility, independent observations in other laboratories) to devise an explanation of how gases combine--the resulting Law of Combining Volumes was announced at a meeting of the Societe Philomatique in Paris on December 31, 1808.    For Gay-Lussac, himself , the law provided a vindication of his belief in regularities in the physical world, which it was the business of the scientist to discover.   These neat ratios do not, however, correspond exactly to his experimental results. He deduced his law from a few fairly clear cases... and glossed over discrepancies in some of the others. The simple reaction between hydrogen and chlorine, which is often used today as an elementary illustration of the law, was not discovered until 1809 and was included only as a footnote when this memoir was printed.
Now comes the pitfall; not Gay-Lussac's at first but John Dalton's. In the second part of his "New System of Chemical Philosophy" Dalton criticized the accuracy of Gay-Lussac's measurements, experiments and generalizations. This was ironic since Dalton was more speculator than experimentalist, sometimes accepting large standard deviations, as in the case of the atomic weight of sulfur, for which he accepted values ranging from 12 to 22 based on his own experiments. Nevertheless, he had the gall to claim that the ratio of volumes of H and O in water was 1:1.97 which, he said, was not a simple whole number ratio, thereby invalidating Gay-Lussac's Law. Thus he was unable to "admit the French doctrine" as he called it.
It is instructive to trace Dalton's thought processes. In a paper read before the Literary and Philosophical Society in Manchester in 1802 (before the Law of Combining Volumes) Dalton stated that: "The particles of one gas are not elastic or repulsive to the particles of another gas but only to the particles of their own kind." In his New System of Chemical Philosophy, Part I (in 1808 after Gay-Lussac's Law), he set out a number of rules for combinations of atoms: "When only one combination of two bodies can be obtained, it must be presumed to be a binary one, unless some cause appear to the contrary". Consequently, hydrogen peroxide not yet having been discovered (it was isolated by Thenard in 1818), Dalton was forced to conclude that the formula of water was HO. Although others of his contemporaries, including Berzelius and Avogadro were quite comfortable with Gay-Lussac's Law and used it to their advantage, Dalton stubbornly rejected it. Like atoms could not stick together. They would repel each other as like charges do. Furthermore, atoms combined in simple proportions by weight according to Dalton's Law of Multiple Proportions, and Dalton could not see how the same proportions could apply to combining volumes. Avogadro had the answer: equal volumes of gases at the same temperature and pressure contain the same number of particles. To the latter, who had used electricity extensively in his studies of the halogens, it must have seemed preposterous to believe that gases such as oxygen, hydrogen and chlorine could be diatomic in nature. Two like atoms and two like charges were bound to repel each other; even though a cornerstone of Avogadro's work was the production of two volumes of HCl from one each of hydrogen and chlorine. So Avogadro's work was consigned to obscurity for fifty years, until his compatriot, Cannizzaro brought it to light in a pamphlet distributed at the end of the Karlsruhe Conference in December 1860 after Gay-Lussac, Berzelius and Dalton were dead.
In the end Dalton did bow under the weight of the evidence and accept the Law of Combining Volumes. But neither he nor Gay-Lussac nor Berzelius ever accepted Avogadro's Law; not even when Gaudin in 1833 clearly showed how it could be applied to explain the formation of water, not even with the background of Cavendish eudiometer experiments.
The stubborn blindness of Dalton and Berzelius and Gay-Lussac is a clear example of a common pitfall in the practice of science. Roger Bacon might have recognized it as the third "Cause of Error": popular prejudice, but it also has elements of Bacon's first cause of error, namely, submission to faulty and unworthy authority. We see that science has the pitfalls of human frailty as well as beauty and nobility.