the soil becomes darkened in proportion. Humic acid in black soil is almost exclusively in combination with lime. A more common view of the difference between the formation of humus above and below the water-level is that above the water-level there is a very free access of air and even the harder parts of the leaf skeleton can be oxidized through the agency of bacteria, while under the water-level there is a very limited supply of air and this oxidation cannot proceed as rapidly. The harder parts of the leaf skeleton are preserved, and from the freer access of air humus is oxidized more readily in drained and open soils, and accumulates in clay soils where there is less circulation of air. The real composition of humus is a matter which has never been definitely determined. Composed of many different but closely related substances it has been difficult to isolate and determine them. Stockbridge[37] gives the following composition of the bodies which form the larger part of humus: Ulmin and Ulmic Acid. Carbon 67.1 per cent Corresponding to C₄₀H₂₈O₁₂ + H₂O. Hydrogen 4.2 „ „ Oxygen 8.7 „ „ Humin and Humic Acid. Carbon 64.4 per cent Corresponding to C₂₁H₂₄_O₁₂ + 3H₂O Hydrogen 4.3 „ „ Oxygen 31.3 „ „ Crenic Acid. Carbon 44.0 per cent Corresponding to C₁₂H₁₂O₈? Hydrogen 5.5 „ „ Nitrogen 3.9 „ „ Oxygen 46.6 „ „ Apocrenic Acid. Carbon 34.4 per cent Corresponding to C₂₄H₂₄O₁₂? Hydrogen 3.5 „ „ Nitrogen 3.0 „ „ Oxygen 39.1 „ „ He further states that there are, aside from these humus compounds, others still less known and the action of which is not yet understood; among them xylic acid, C₂₄H₃₀O₁₇, saccharic acid, C₁₄H₁₈O₁₁, glucinic acid, C₁₂H₂₂O₁₂, besides a brown humus acid containing carbon, 65.8 per cent, and hydrogen, 6.25 per cent, and a black humus acid yielding carbon, 71.5 per cent, and hydrogen, 5.8 per cent. According to Mulder humic acid has the following composition, C₆₀H₅₄O₂₇, while Thenard[38] ascribes to it the formula, C₂₄H₁₀O₁₀. At the present time we can only regard the various forms of humus bodies as mixtures of many substances mostly of an acid nature and resulting from a gradual decomposition of organic matter under conditions which partially exclude free access of oxygen. For analytical purposes it is only necessary to separate these bodies by the best approved processes. A further knowledge of their composition can then be derived by determining the percentages of carbon dioxid and water which they yield on combustion.