Chlorine is a deadly poison gas employed on European battlefields in World War I. Sodium is a corrosive metal which burns upon contact with water. Together they make a placid and unpoisonous material, table salt. Why each of these substances has the properties it does is a subject called chemistry.

From common salt are obtained chemically as primary derivatives chlorine—both a war gas and a means of purifying water; and 'caustic soda.' … [O]n the chlorine side there is obtained chloride of lime, (a bleaching powder and a disinfectant), chloroform (an anesthetic), phosgene (a frightful ware gas), chloroacetophenone (another war gas), and an indigo and a yellow dye. [O]n the soda side we get metallic sodium, from which are derived sodium cyanide (a disinfectant), two medicines with [long] names, another war gas, and a beautiful violet dye. Thus, from a healthful, preservative condiment come things useful and hurtful—according to the intent or purpose.

The cure for anything is salt water: sweat, tears or the sea.

[Professor Bragg asserts that] In sodium chloride there appear to be no molecules represented by NaCl. The equality in number of sodium and chlorine atoms is arrived at by a chess-board pattern of these atoms; it is a result of geometry and not of a pairing- off of the atoms.