1808 – France

‘Volumes of gases which combine or which are produced in chemical reactions are always in the ratio of small whole numbers’

One volume of nitrogen and three volumes of hydrogen produce two volumes of ammonia. These volumes are in the whole number ratio of 1:3:2

# N_{2} + 3H_{2} ↔ 2NH_{3}

Along with his compatriot Louis Thenard, Gay-Lussac proved LAVOISIER’s assumption that all acids had to contain oxygen, to be wrong.

Gay-Lussac reexamined JACQUES CHARLES’ unpublished and little known work describing the phenomenon that the volume of a gas at constant pressure is directly proportional to temperature – and ensured that Charles received due credit for his discovery.

Alongside JOHN DALTON, Gay-Lussac concluded that once pressure was kept fixed, near zero degrees Celsius all gases increased in volume by 1/273 the original value for every degree Celsius rise in temperature. At 10degrees, the volume would become 283/273 of its original value and at -10degrees it would be 263/273 of that same original value. He extended this relation by showing that when volume was kept fixed, the pressure exerted on the walls of a container of gas would increase or decrease by the same 1/273 factor when temperature was shifted by a degree Celsius. This did not depend upon the gas being studied and hinted at a deep connection shared by all gases. If the volume of a gas at fixed pressure decreased by 1/273 for every 1degree drop, it would reach zero volume at -273degrees Celsius. The same was true for pressure at fixed volume. That had to be the end of the scale, the lowest possible temperature one could reach. Absolute zero.

Gay-Lussac was an experimentalist and his conclusions were based on the results of extensive experiments. In an 1807 gas-experiment, Gay-Lussac took a large container with a removable divider down the middle and filled half with gas and made the other half a vacuüm. When the divider was suddenly removed, the gas quickly filled the whole container. According to caloric theory, temperature was a measure of the concentration of caloric fluid and removal of the divider should have led to a drop in temperature because the fluid was spread out over a greater volume without any loss of caloric fluid. (The same amount of fluid in a larger container means lower concentration).

Evidence linking heat to mechanical energy accumulated. Expenditure of the latter seemed to lead to the former.

The explanation of why gases combine in small-number ratios came from AVOGADRO.