"Chemists of the Bureau of Mines in the course of an investigation of methods of preventing explosions in coal mines by the use of rock dust had to determine accurately small percentages of combustible matter (organic carbon and organic hydrogen) in clay, shale, limestone, and other rocks for use in rock-dusting mines. Owing to the presence of combined water in shale and clay, and of carbon dioxide in carbonate rocks, the percentage of combustible matter in these materials can not be calculated from the loss on ignition, as is done in coal and other fuels containing a relatively small proportion of constituents that are incombustible; neither can the combustible be taken as the sum of the total carbon and hydrogen as obtained in the ordinary ultimate analysis, for in both these methods the organic matter is oxidized to water and carbon dioxide, simultaneously with the liberation of water and carbon dioxide from the hydrous silicates and inorganic carbonates in the rock. Therefore, special methods are required that will differentiate between the organic and the inorganic carbon and hydrogen."