Joseph Black was first to use a balance to follow, and thereby rationalise, chemical reactions. The letter is written to James Smithson (1765–1829), whose bequest established the Smithsonian Institution. The letter describes a light beam of fir-wood, resting across a fine needle balanced over a small brass holder. The assembly would lie only about 4 mm above the table. Black writes “I can learn the weight of any little mass, from 1 grain or a little more to the 1-1200th of a grain”, which in modern units corresponds to 65 mg down to 0.05 mg.