3."That in lieu of the stamp duties chargeable on certificates of registration of the works mentioned in sub-section (1) of section nine of the Alkali, etc., Works Regulation Act, 1906, there shall be charged

time while such licence is in force used in an altered condition or in a manner or for a purpose which brings it within, or which if it was used solely in that condition or in that manner or for that purpose, would bring it within a class or description of vehicles to which a higher rate of duty is applicable under the said Schedule, duty at such higher rate shall become chargeable in respect of the licence for the vehicle."