The staining of the particles after ignition and treatment with acids gives better results than the direct treatment. Particles of carbonate which are stained with difficulty before ignition take the stain easily afterwards on account of the decomposition produced by the loss of carbon dioxid. This is the case also with particles containing water of composition or crystallization. =257. Cleavage of Soil Particles.=—A microscopic examination of the cleavage of soil particles may be useful in determining their mineral origin. The course followed by cleavage lines and their mutual position is dependent on the direction in which the separation of the mineral fragment takes place. The character of the microscopic fragments produced by crushing a soil particle is determined primarily by the system of crystallization to which it belongs. Perhaps the most distinguishing cleavage marks in soil particles will be found in fragments of mica and orthoclase. These characteristic forms are shown in Figs. 39 and 40. The first (Fig. 39) shows the pinacoidal cleavage in a fragment of mica. Fig. 40 illustrates the appearance of the cleavage lines in a fragment of orthoclase. Figs. 41 and 42 show the characteristic cleavage lines in fragments of epidote and titanite. =258. Microchemical Examination of Silt.=—The methods of quantitative chemical examination of silts will be given in another part of this manual. Certain qualitative and microchemical tests, however, are useful in identifying silt particles. For instance, any soluble iron mineral will be detected, even in minute quantity, by the blue coloration of the solution produced by the addition of potassium ferrocyanid. Manganese will be revealed by fusion with soda and saltpeter on platinum foil, in the oxidizing flame, producing the well-known green coloration due to the sodium manganate formed. More valuable indications of the character of the fragments examined are obtained by microchemical processes. The best method of decomposing the silt particles for this purpose is by treatment with hydrofluosilicic acid. When the particles are composed of silicates, pure hydrofluoric acid is to be preferred.