From Leonard Darwin   8 January 1878

Dear Father

Today I tested that lamp black for ammonia.¹ In doing this test, the substance is put into a still and the water that distills over is tested for the ammonia. A good deal of greasy matter came over, shewing the presence of oil or fat in the lamp black. The amount of ammonia present proved to be 0^.1 to 0^.2 per cent of the lamp-black. This is free ammonia, or ammonia in any salt. The test is not a very sure one, for some organic substances, of which urea is the only one I know of, test as if they were ammonia. But there is sure not to be more than ^.1 to ^.2 per cent of ammonia present. Carbon absorbs ammonia readily, and I think it would take up this amount, if the air near it had ever been charged with ammonia; as it is sure to be in a laboratory sometimes.

Your affec son | L. Darwin.