(b)"unabsorbed depreciation" means so much of the allowance for depreciation of the predecessor firm or the proprietary concern or the amalgamating company or the demerged company, as the case may be, which remains to be allowed and which would have been allowed to the predecessor firm or the proprietary concern or amalgamating company or demerged company, as the case may be, under the provisions of this Act, if the reorganisation of business or amalgamation or demerger had not taken place.'.