All oils sold under the names of sperm, summer, fall and winter oils are deemed to be sold for pure sperm oil, the test of which is Southworth's oleometer. Whoever sells under said names any oils which are adulterated by the mixture of an inferior article, without disclosing the full extent of adulteration to the purchaser, forfeits to the prosecutor $15 for each offense. The oil so sold shall be deemed whale oil, and the seller is liable to the purchaser for the difference between pure sperm oil and whale oil, to be recovered in a civil action.