(1) Save as hereinafter provided in this section, it shall be no defence in a prosecution under this Chapter to prove merely that the accused was ignorant of the nature, sub stance or quality of the drug [or cosmetic] in respect of which the offence has been committed or of the circumstances of its manufacture or import, or that a purchaser, having bought only for the purpose of test or analysis, has not been prejudiced by the sale.
(2) [For the purposes of section 18 a drug shall not be deemed to be misbranded or [adulterated or spurious] or to be below standard quality nor shall a cosmetic be deemed to be misbranded or to be below standard quality] only by reason of the fact that--
(a) there has been added thereto some innocuous substance or ingredient because the same is required for the manufacture or preparation of the drug [or cosmetic] as an article of commerce in a state fit for carriage or consumption, and not to increase the bulk, weight or measure of the drug [or cosmetic] or to conceal its inferior quality or other defects; or
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(b) in the process of manufacture, preparation or conveyance some extraneous substance has unavoidably become intermixed with it: provided that this clause shall not apply in relation to any sale or distribution of the drug [or cosmetic] occurring after the vendor or distributor became aware of such intermixture.
[(3) A person, not being the manufacturer of a drug or cosmetic or his agent for the distribution thereof, shall not be liable for a contravention of section 18 if he proves--
(a) that he acquired the drug or cosmetic from a duly licensed manufacturer, distributor or dealer thereof;
(b) that he did not know and could not, with reasonable diligence, have ascertained that the drug or cosmetic in any way contravened the provisions of that section; and
(c) that the drug or cosmetic, while in his possession, was properly stored a nd remained in the same state as when he acquired it.]