Sentence:  On the Continent, where common salt is heavily taxed, alum, carbolic acid, naphthalene and other materials are frequently added to it to “<WSD>denaturise</WSD>,” or render it incapable of being used as food, and these additions are often the cause of trouble to the tanner. 
Meaning of the word marked with <WSD> tags:  Adulterate (alcohol) to make unsuitable for human consumption