You are likely asking about the chemical used to detect leaks in gas cylinders. There isn't one specific chemical, but the most common method involves using soap solution or a commercially available leak detection solution.

Here's why:

* Soap solution: When applied to a suspected leak, the escaping gas will create bubbles. This visual indication is a clear sign of a leak. This method is particularly effective for leaks of flammable gases like propane, butane, and natural gas.

* Commercially available leak detection solutions: These solutions contain specialized chemicals that react with the escaping gas, producing a visible or olfactory indication of a leak. They are often formulated for specific gas types and can offer greater sensitivity compared to simple soap solution.

Important Note:

* It's crucial to use leak detection solutions specifically designed for the gas you are testing. Using the wrong solution could be dangerous or ineffective.

* Always follow the manufacturer's instructions for using leak detection solutions.

* Never use open flames or sources of ignition while checking for leaks.

Additionally:

* Electronic leak detectors are also available, but they are more expensive and may not be as widely accessible as soap solution. These detectors use sensors to detect the presence of specific gases.

Remember, safety should always be your top priority when dealing with gas cylinders. If you're unsure about how to check for leaks, consult a qualified professional.