The system of lead cotton
In ancient times, lead cotton was used for gas testing. The method is to take the acetate of lead and make it into a cotton, which is lead cotton.

Whenever there is something without sulfur in the gas to be tested, put the lead cotton in the gas. If the gas contains sulfur, the color of the lead cotton changes. The sulfur cover combines with the acetate phase of lead to produce black lead sulfide, and the lead cotton turns from white to dark.

All places where the gas is tested, such as mines, smelters, or places where there is a gas leak, can be tested with lead cotton. If the color of the lead cotton changes, you will know whether the gas is pure or not. Although this lead cotton is a small thing, it has a great function in testing qi. It can make people predict the harm of qi and take precautions early to protect themselves and keep their careers smooth.