From Leonard Darwin   7 January 1878

Dear Father

The following is the work on the lamp black so far. Boiled 25 grains in water; filtered; obtained a clear solution with a very slight green tint; evaporated to dryness and a residue was left; this may have been oil that came through with the hot water; then I red heated the residue, and a good deal went away with badly smelling fumes; there only remained .08 grain   I think that this contained a trace of iron; and that most of it was some soda salt, but I cannot be sure.¹ The result in numbers is

Your affec son | L. Darwin.