This paper described the composition of some crystallized slags from Birtley Iron-Works, in the County of Durham. These slags, like those from Wales described by Professor Miller in the Cambridge Transactions, had the form of the olivine, and in composition were nearly pure silicate of iron, containing about 0.4 per cent, of foreign matter, chiefly magnesia. The author also gave a short account of a method by which the magnet might be made available in giving immediately very near approximations to the quantity of iron contained in the basic silicates of iron.