He

1. Helium can be used as a pressurizing and pressurizing agent for liquid rocket fuel, and is used in large quantities in missiles, spacecraft and supersonic aircraft.

2. Helium is used as a protective gas in smelting and welding, which is very important in shipbuilding as well as in the manufacture of airplanes, spacecraft, rockets and weapons. 

3. Helium has excellent permeability, used for nuclear reactor cooling, rockets and nuclear reactors, some pipelines and electronic and electrical devices such as leak detection.

4. Helium is a gas with ideal gas properties, is the ideal gas for the vapor pressure thermometer at very low temperatures.

5. Helium's mass density, weight density are low, and non-flammable, can be used to fill light bulbs, neon tubes, but also ideal for balloons and airships with gas.

6. Liquid helium can be obtained close to the absolute temperature (-273 ℃) of the low temperature, used in the manufacture of superconducting equipment.

7. Helium inert gas of a kind, the solubility in the blood is lower than nitrogen, and thus its anesthesia is lower than that of nitrogen, so helium is often mixed with oxygen, as a diver's breathing gas.