Excise exemption for limestone captively used in cement manufacture relieves duty where non-levy occurred under prior practice. The Central Government directs that where, in accordance with an established practice, excise duty was not levied on limestone of Heading 25.05 captively consumed in cement clinker manufacture during the relevant period, the full excise duty that would otherwise have been payable need not be paid, thereby recognizing the prior non-levy and relieving liable persons of payment for that period.