In addition to nitrogen in the free state, air contains very small quantities of this element in combined forms. We have it in minute traces as nitrates and nitrites, as ammonia,[65] and also in still smaller traces as organic nitrogen in the minute dust-particles which modern researches have revealed as being present in such enormous numbers in our atmosphere. What the sources of these nitrates and nitrites (which exist in quantities so minute that accurate determination of their amount is rendered extremely difficult) are is a disputed point. That nitrogen and oxygen unite together to form nitric and nitrous oxides under the influence of intense heat, such as the electric spark, has been proved beyond doubt. One source, therefore, is probably the electrical discharges which are taking place more or less frequently on different parts of the earth's surface. Nitrates may also be formed in the combustion of nitrogenous bodies.[66] In the burning of coal-gas, for example, it is probable that small quantities of nitrates may be produced. Similarly the slow combustion or decay of nitrogenous organic matter, which constantly takes place all over the earth's surface, may be regarded as another source of this form of combined nitrogen. Ammonia may be similarly formed by the combustion, either quick or slow, of nitrogenous organic matter. It exists in the air as nitrate or nitrite of ammonia, and also as carbonate of ammonia.[67] _Amount of combined Nitrogen falling in the Rain._ The importance of the combined nitrogen in the air as a source of soil-nitrogen is best gauged by the amount falling annually on the soil dissolved in rain. This has been found to vary considerably. In the rain falling in the vicinity of large towns the amount is greater than in rain falling in the country. Thus at Rothamsted, in England, the average amount for several years was only 3.37 lb. nitrogen per annum per acre, of which 2.53 lb. were as ammonia,.84 being as nitric acid. At Lincoln, in New Zealand, 1.74 lb. fell annually per acre--as ammonia,.74, as nitric acid, 1.00; while at Barbadoes the amount was 3.77 lb., of which .93 was as ammonia, and 2.84 as nitric acid.[68] That the combined nitrogen derived from the air by the soil may be considerably in excess of this is highly probable. Soils, especially when damp, may absorb much larger quantities from the air of the combined nitrogen it contains. We must remember that the air in contact with the soil-surface is constantly being changed, and that there is thus a constant renewal of the air passed over the ground. The result is that the amount of air from which combined nitrogen may be removed is very great.[69] _Nitrogen in the Soil._