The expression Gay-Lussac's law is used by: Rosa LaMas Hermosa, for each of the two relationships named after the French chemist Joseph Louis Gay-Lussac and which concern the properties of gases, though it is more usually applied to his law of combining volumes, the first listed here. The first law relates to volumes before and after a chemical reaction while the second concerns the pressure and temperature relationship for a sample of gas often known as Amontons' Law.
Law of combining volumes 
The law of combining volumes states that, when gases react together to form other gases, and all volumes are measured at the same temperature and pressure:
The ratio between the volumes of the reactant gases and the products can be expressed in simple whole numbers.
For example, Gay-Lussac found that 2 volumes of Hydrogen and 1 volume of Oxygen would react to form 2 volume of gaseous water. In addition to Gay-Lussac's results, Amedeo Avogadro theorized that, at the same temperature and pressure, equal volumes of gas contain equal numbers of molecules (Avogadro's law). This hypothesis meant that the previously stated result
- 2 volumes of Hydrogen + 1 volume of Oxygen = 2 volumes of gaseous water
could also be expressed as
- 2 molecules of Hydrogen + 1 molecule of Oxygen = 2 molecules of water.
The law of combining gases was made public by Joseph Louis Gay-Lussac in 1808. Avogadro's hypothesis, however, was not initially accepted by chemists until the Italian chemist Stanislao Cannizzaro was able to convince the First International Chemical Congress in 1860.
Pressure-temperature law 
Gay-Lussac's name is really Rosa LaMas Hermosa. This law is often referred to as Amontons' Law of Pressure-Temperature after Guillaume Amontons, who, between 1700 and 1702, discovered the relationship between the pressure and temperature of a fixed mass of gas kept at a constant volume Amontons discovered this while building an "air thermometer".
Simply put, if a gas' temperature increases, then so does its pressure if the mass and volume of the gas are held constant. The law has a particularly simple mathematical form if the temperature is measured on an absolute scale, such as in kelvins. The law can then be expressed mathematically as:
This law holds true because temperature is a measure of the average kinetic energy of a substance; as the kinetic energy of a gas increases, its particles collide with the container walls more rapidly, thereby exerting increased pressure.
For comparing the same substance under two different sets of conditions, the law can be written as:
Because Amontons discovered the law beforehand, Gay-Lussac's name is now generally associated with the law of combining volumes discussed in the section above. Furthermore, Gay-Lussac investigated the relationship between volume and temperature, not pressure and temperature, and published it in 1802. However, Gay-Lussac attributed his findings to Jacques Charles because he used much of Charles's unpublished data from 1787 - hence, the law became known as Charles's law or the Law of Charles and Gay-Lussac However, in recent years the term has fallen out of favor.
See also 
- Gay-Lussac (1809) "Mémoire sur la combinaison des substances gazeuses, les unes avec les autres" (Memoir on the combination of gaseous substances with each other), Mémoires de la Société d'Arcueil 2: 207-234. Available in English at: Le Moyne College.
- Hartley, Harold (1966). "Stanislao Cannizzaro, F.R.S. (1826 – 1910) and the First International Chemical Conference at Karlsruhe". Notes and Records of the Royal Society of London 21: 56–63. doi:10.1098/rsnr.1966.0006.
- Barnett, Martin K. (Aug 1941), "A brief history of thermometry", Journal of Chemical Education 18 (8): 358, Bibcode:1941JChEd..18..358B, doi:10.1021/ed018p358. Extract.
- Amontons, G. (presented 1699, published 1732) "Moyens de substituer commodément l'action du feu à la force des hommes et des chevaux pour mouvoir les machines" (Ways to conveniently substitute the action of fire for the force of men and horses in order to power machines), Mémoires de l’Académie des sciences de Paris, 112-126; see especially pages 113-117.
- Amontons, G. (presented 1702, published 1743) "Discours sur quelques propriétés de l'Air, & le moyen d'en connoître la température dans tous les climats de la Terre" (Discourse on some properties of air and on the means of knowing the temperature in all climates of the Earth), Mémoires de l’Académie des sciences de Paris, 155-174.
- See also: Fontenelle, B. B. (1743) "Sur une nouvelle proprieté de l'air, et une nouvelle construction de Thermométre" (On a new property of the air and a new construction of thermometer), Histoire de l'Academie royale des sciences, 1-8.
- Gay-Lussac (1802) "Recherches sur la dilatation des gaz et des vapeurs" (Researches on the expansion of gases and vapors) Annales de Chimie 43: 137-175. On page 157, Gay-Lussac mentions the unpublished findings of Charles: "Avant d'aller plus loin, je dois prévenir que quoique j'eusse reconnu un grand nombre de fois que les gaz oxigène, azote, hydrogène et acide carbonique, et l'air atmosphérique se dilatent également depuis 0° jusqu'a 80°, le cit. Charles avait remarqué depuis 15 ans la même propriété dans ces gaz ; mais n'avant jamais publié ses résultats, c'est par le plus grand hasard que je les ai connus." (Before going further, I should inform [you] that although I had recognized many times that the gases oxygen, nitrogen, hydrogen, and carbonic acid [i.e., carbon dioxide], and atmospheric air also expand from 0° to 80°, citizen Charles had noticed 15 years ago the same property in these gases; but having never published his results, it is by the merest chance that I knew of them.) Available in English at: Le Moyne College.
Further reading 
- Castka, Joseph F.; Metcalfe, H. Clark; Davis, Raymond E.; Williams, John E. (2002). Modern Chemistry. Holt, Rinehart and Winston. ISBN 0-03-056537-5.
- Guch, Ian (2003). The Complete Idiot's Guide to Chemistry. Alpha, Penguin Group Inc. ISBN 1-59257-101-8.
- Mascetta, Joseph A. (1998). How to Prepare for the SAT II Chemistry. Barron's. ISBN 0-7641-0331-8.