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23. Another group embraces casing head gasoline, natural gas and liquid petroleum gas. Though bitumen itself is a hydrocarbon, it has a complex hydrocarbon structure with traces of vanadium, nickel, nitrogen and sulfur.

24. At the relevant time the tariff required that for assessment under item 10, the oil should have, amongst other things, one quarter of one per cent or more by weight of bituminous substances. We have seen that the same is also required of goods to be assessed under item 9 as diesel oil not otherwise specified. The test was later changed to a test for carbon residue of not less than one quarter of one per cent by weight when tested by Rams-bottom carbon residue apparatus. We have seen also the Chemical Examiner of the Custom House saying that there was no direct method for qualitative estimation of the bituminous substance content. Indeed, to our knowledge there is no method by which the quantity of bituminous substances in a petroleum product can be directly determined and this was quite true even before the Chemical Examiner wrote his note dated 7.12.1976. But it appears that the laboratory had been reporting a quantity of bituminous content in petroleum products for a long time, doing so in accordance with the Board's instructions of November, 1976. Surety, it was known to the laboratory that a carbon residue is not a result only of bituminous substances in the oil tested but of various other substances, although bituminous substances would leave relatively more carbon residue than other petroleum derived substances. We have not been told the nature of the qualitative test which the Custom House devised. Qualitative tests generally employed are the nuclear magnetic resonance tests; quantitative methods are gravimetric and volumetric methods, as, for example, precipitation, titration and solvent extraction. In qualitative analysis the aim is chiefly identification of materials. In a product like Dutrex RT, qualitative tests like nuclear magnetic resonance and nuclear quadrupole resonance etc. can reveal the existence of bituminous substances. But Dutrex RT does not appear to be a mixture, and, therefore, it is doubtful if qualitative tests, apart from merely detecting the presence of trace elements, can determine the volume or percentage or the weight of the element sought to be identified. The Chemist himself says in his note of December, 1976 that they evolved a simple qualitative test for verifying the presence of bituminous substances; the tests did not appear to have aimed at determining the percentage of the bituminous substances. All the test reports made in accordance with this test certify only the absence of bituminous substances. In his note Dr. Rajamani of the Jawaharlal Nehru University points out that the absence of bituminous substances can be proved by physical techniques such as NMR, X-Ray diffraction and moleculer weight determination or by chemical analysis for vanadium, sulfur, nitrogen. This, in our opinion, is the key to the whole matter and fits in with the statement of the Chemical Examiner of the Madras Custom House that there was no quantitative tests for determining bituminous substances content in a mineral oil. We will record here briefly that in his note of December, 1976, the Chemical Examiner Madras threw doubts not only of the carbon residue test prescribed by the Board in 1956, but also the colour of the oil when compared with an 0.04 N Iodine solution. He apparently did nothing about this but concentrated only on deciding a qualitative test in place of the Ramsbottom carbon residue test.