Why did Antoine Lavoisier weigh gases as well as solid substances?

Antoine Lavoisier weighed gases and solid substances to demonstrate the conservation of mass in chemical reactions, asserting that matter is neither created nor destroyed. By meticulously tracking the weights of all substances entering and emerging from his experiments, he sought to elevate chemistry to the rigorous investigative standards of experimental physics.12

His focus on weighing gases was critical to his research on combustion, respiration, and calcination. Lavoisier observed that when metals were converted into powders (calxes), they gained weight, while the reduction of calxes to metal resulted in a weight loss. He hypothesized that these changes were caused by the fixation and release of gas rather than the loss of "phlogiston," a substance that could not be weighed. By accounting for the weight of the gases involved, he successfully identified oxygen as the active entity in these processes, effectively replacing the phlogiston theory and establishing a foundation for modern chemistry.32