6. (1) No person shall, directly or indirectly and whether by himself or by any other person acting on his behalf,- (a) manufacture or sell any article of food which is adulterated, or (b) sell to the prejudice of the purchaser any article of food which is not of the nature, substance or quality demanded by the purchaser. (2) An offence shall not be deemed to have been committed under sub-section (1), if the article of food contains the normal constituents prescribed under clause (a) of sub-section (1) of section 5, and if the chemical and physical constants of the article are in accordance with those referred to in the said clause- (a) where any innocuous substance or ingredient has been added thereto, if such substance or ingredient- (i) is required for the production or preparation of such article as an article of commerce in a condition fit for carriage or consumption, and (ii) is not so added fraudulently to increase the bulk weight or measure, or to conceal the inferior quality, of such article: Provided that the admixture of such substance or ingredient does not render such article to be injurious to health; or(b) where any constituent has been abstracted therefrom if such abstraction is required for the production or preparation of such article as an article of commerce fit for carriage or consumption and does not render such article to be injurious to health; or (c) where a patent has been granted under any law for the time being in force, if such article is manufactured or sold in the condition required by the specification of the patent. (3) The opinion of the public analyst on the question whether any substance or ingredient is innocuous shall, subject to the provisions of section 32, be conclusive evidence on such question. (4) In any prosecution under this section,- (a) it shall not be a defence to allege- (i) that the purchaser bought for analysis or examination and therefore was not prejudiced, or (ii) that the person who manufactured or sold the article of food had no knowledge of, and could not with reasonable diligence have ascertained, its nature, substance or quality; and (b) the Court shall, until the contrary is proved, presume that any article of food, which is found in the possession of any person who manufactures or sells similar articles, has been manufactured by that person, or is for sale by him, as the case may be.