These petitions were heard together because common questions of fact and of law arise in them for consideration. They are filed by certain manufacturers of patent or proprietary medicines under Article 226 of the Constitution for writs in the nature of mandamus or certiorari, as the case may be. In all these cases the respondents who are the Board of Revenue, Authorities of the Commercial Tax Department and the District Revenue Officer charged with the administration of the Medicinal and Toilet Preparations Excise Duties Act, 1955 (Central Act XVI of 1955) hereinafter called the Act, as amended in 1961 have, after calling for returns of their manufacture of patent and proprietary medicines since 1st June, 1961, when an amendment to the aforesaid Act was brought about under Act XIX of 1961, issued demands against the petitioners for payment of excise duty calculated ad valorem at 10 per cent, of the value on the patent or proprietary medicines manufactured by them subsequent to 1st June, 1961.

The first contention of the petitioners in these cases, for challenging the correctness and validity of the above demand, was that under the Schedule to Act XVI of 1955, in item 1, und