5. (1) The Government may determine by rules in this behalf- (a) the normal constituents and the values of the chemical and physical constants of any article of food, and(b) in respect of a sample of any article of food, the deficiency in any such constituent or the addition of any extraneous matter or the proportion of water which shall, for the purpose of this Ordinance and until the contrary is proved, raise a presumption that such article is adulterated.(2) A public analyst shall, when certifying the result of any analysis made under this Ordinance, have regard to the rules referred to in sub-section (1). (3) When any rule referred to in sub-section (1) has been made, the purchaser of any article of food to which rule applies shall, until the contrary is proved, be deemed to have demanded food complying with such rule.