What is the Law of Conservation of Mass?June 26, 2021
Speaking of his experiments on mercuric oxide, Priestley states the properties of the “competent gas” he found. Lavoisier does not give priority to Priestley in any of his publications; he simply says once, “We discovered oxygen at about the same time as Priestley.”
In fact, Lavoisier was not the first to discover oxygen; but he was the first scientist to grasp the true significance of this gas. He sets out to repeat Priestley’s experiments with peculiar care and attention. It is seen that when mercury is heated in a container containing a certain amount of air, it gains weight by turning the mercury into red mercury oxide, while the air loses weight to the same extent. Lavoisier goes one step further in his experiments: after weighing the mercury oxide (calx), which it separates from the mercury, it heats it further; He detects that the red oxide, which turns into ember, is gradually disappearing, and that with a certain number of mercury particles, a more effective amount of “elastic fluid” than atmospheric air remains in the respiration and combustion processes. The elastic fluid was what Priestley called “competent gas”.
Lavoisier further determines that the weight of this residue is equal to the weight of air, which is reduced by the initial heating of the mercury. Moreover, the weight lost by the conversion of mercury oxide into mercury under heat occurs when it combines with the effective part (ie, oxygen). This theory, which was ignored at first, did not delay in attracting the attention of the scientific community when Cavendish announced the results of the experiment that water was formed by the combination of two gases. In his experiments, Cavendish obtained a gas he called “flammable” from the effects of acids on the metal, which he thought was phlogiston. But an experiment by Priestley frees him from this misinterpretation. Priestley saw some dew form when a mixture of hydrogen and oxygen ignited a gas with an electric spark. Repeating the same experiment, Cavendish goes further and finds that all of the “flammable” gas and one-fifth of normal air were consumed in the explosion, and that the dew formed was pure water.
The phlogiston theory was overturned. The adoption of the new theory did not take long, despite the resistance of some bigoted circles. The delayed breakthrough in chemistry is finally realized. Lavoisier presents his conclusion to the Academy of Sciences in a paper; however, in a word, Priestley, Cavendish, etc. does not mention the contributions of the experimenters. It can be said that Lavoisier discovered neither a new chemical object nor a new chemical phenomenon. His aim was to establish a new and working system. His Traité Élémentaire de Chimie, published in 1789, is Newton’s Principia in its field. One laid the foundation for modern physics, the other for modern chemistry.