RAMASWAMI, J.

T.C. Nos. 298 to 300 and 314 of 1971 relate to the assessment years 1962-63 to 1965-66 and T.C. No. 305 of 1971 relates to the assessment year 1966-67. The assessee in all these cases is the same. The assessees are dealers in fertilisers, manure mixtures and rock phosphate, etc. They purchased number of items of chemical fertilisers both locally and also from outside the State. In some cases, they mixed organic chemicals in various proportions and manufactured different types of mixture manures called "tea manure", " coffee manure", etc. Under item 21 of the First Schedule to the Tamil Nadu General Sales Tax Act, the chemical fertilisers enumerated in that item are to be taxed at single point on the point of first sale in the State. After enumerating 15 items of chemical fertilisers, the entry further provided that any mixture of one or more of the articles mentioned therein with one or more of the organic manures would also come within that entry. In the assessment years 1962-63 to 1965-66, the assessee claimed exemption in respect of the mixture manures of the ground that the manufacture of manure mixtures does not involve any manufacturing process and does not also involve any chemical changes of the organic ingredients and that, therefore, they are not commercially different goods. On that basis, the assessee claimed that the sale of manure mixture is a second sale in the State as it was made of the organic chemicals purchased by them. The assessing authority found from a check of the records that the assessee had not only purchased organic chemical fertilisers within the State but also a good portion of the turnover related to purchase from outside the State. In respect of the chemicals purchased by the assessee from outside the State, the assessing officer considered that the sale of the same by the assessee would be the first sale within the State and that, therefor, under entry 21 that turnover is liable to be taxed. He also found that the assessee had not been keeping separate accounts for the first and subsequent sales and he accordingly proposed to estimate the first sales with reference to a formula.The formula determined by "local purchases x local sales". him was

total purchases

On this basis, he determined the turnover relating to the second sales and exempted only that portion and subjected to tax the remaining portion. When the question was again considered for the assessment year 1966-67, the assessing authority thought that in the absence of any evidence to show what was the proportion of the organic chemical which was used in the manufacture of manure mixture, no portion of the sales turnover of manure mixture could be exempted as second sales. In that view, he assessed the entire turnover relating to manure mixture for the assessment year 1966-67. He also initiated proceedings under section 16 in respect of the assessment years 1962-63 to 1965-66 on the ground that that portion of the turnover relating to sales of manure mixture exempted on the earlier occasion had escaped assessment. The assessee preferred appeals both against the reassessment orders made under section 16 and the assessment order made under section 12 in respect of the assessment year 1966-67. The Appellate Assistant Commissioner confirmed the order of the assessing authority. The Tribunal also confirmed the decision except in regard to a small portion, which is not the subject-matter of revision in these proceedings. Before the Appellate Assistant Commissioner and the Appellate Tribunal, the assessee contended that in so far as the assessment years 1962-63 to 1965-66 are concerned, the assessing authority has no jurisdiction to invoke his power under section 16 as the assessing authority on the earlier occasion had considered that question and deliberately exempted the turnover as second sales. In those circumstances, the turnover could not be said to have escaped assessment giving jurisdiction to the assessing aut