for the purpose of mixing these materials together into a homogeneous mixture as illustrated in fig2 the batch of materials piles up in the hopper to a level as designated by solid line 37 with closure member 35 in the lowered position closing off outlet 14 a solenoid controlled valve 38 is operated to permit air from air supply 29 to enter into the cylinder chambers of poppet valves 22 so that each valve piston is driven to its furthest upward position to seal off its associated opening or nozzle into the interior of hopper 19 at the same time air is being supplied from the source 18 into chamber 16 of manifold 15 but at a pressure somewhat lower than that being applied to the piston of poppet valve 22 this holds the valve piston so that it closes the nozzle 20 and no pressurized air is allowed into the interior of the hopper 19 for mixing or blending together the batch of material contained in hopper 19 solenoid valve 38 is operated so that the air pressure applied into the cylinder of poppet valve 22 is reduced below that of the air pressure in chamber 16 so that the piston member is driven away from nozzle 20 and a blast of pressurized air is injected upward into the interior of the hopper 19 through nozzles 20 the.