The petitioners are a firm of building contractors. In December, 1956, they entered into a contract with the Military Engineering Services Department of the Geivernment for the construction of certain buildings known as "Married accommGdation" at Ambala Cantonment and received a sum of Rs. 32,000 on January 31, 1957, as advance. On February 14, 1957, the assessing authority, J ullundur District issue:l a notice intimating the petitioners that as they had failed to apply for registration under s. 7 of the Act assessment would be made under s. 18, sub-s. (2), for the periods commencing from April 1, 1955, onwards, and calling upon them to produce their account books and attend the hearing on February 16, 1957. Thereupon, the petitioners tiled the present petition under Art. 32 of the Constitution challenging the legality of the assessment proceedings, the main ground of attack being that the legislature of the Province .of Punjab had, under Entry 48 in List II of Sch. VII to the Government of India Act, 1935, no pml'er to impose tax on the supply of materials in construction works as there was no sale in fact or' in Jaw of those •