The simplest, lightest gas, which in burning combines with oxygen to produce only water and heat, so is an ideal fuel gas.  But its production and handling entail many problems. It has the highest thermal conductivity of any gas, so can be used for cooling purposes in rotary elecrical generators.  It is detected by catalytic sensors, by metal oxide detectors or by thermal conductivity sensors.  As the nation moves toward a “Hydrogen economy” it becomes more important to all of us.