Abstract: Reaction heat values of basic aluminum sulfate solution with acid such as sulfuric acid are correlative with an amount of base contained in said solution. The difference of temperatures of said solution before and after its reaction with said acid is utilized as an indication for the calibration of the basic amount in the solution. This calibration is effectively utilized for the control of the basic amount of said solution which is employed in the removal of SO.sub.2 gas from a waste gas.

Abstract: A rectangularly shaped tunnel is constructed to include an artificial roof, side wall pillars, a center wall pillar and floor portions all extending along the length of tunnel. The artificial roof is formed by sequentially excavating cross cut shafts and by subsequently plugging the thus formed cavities with concrete. Then the portions directly beneath opposite sides and the center of the roof are excavated and filled with concrete to form side wall pillars and a center wall pillar. Thereafter the rock portions covered by the roof and the wall pillars are excavated to form inner hollow tunnel chambers, the bottoms of which are covered by concrete to form floor portions. Iron frames are embedded in the concrete.