=191. Separation of the Soil Into Particles of Standard Size.=—The agronomic value of a soil depends largely on the relative size of the particles composing it. The finer the particles, within a certain limit, the better the soil. The size of the particles may be estimated in three ways: (1) by passing through sieves of different degrees of fineness; (2) by allowing them to subside for a given time in water at rest; (3) by separating them in water moving at a given rate of speed. The first method is a crude one and is used to prepare in a rough way, the material for the second and third processes. =192. Separation in a Sieve.=—The soil should be dry enough to avoid sticking to the fingers or to prevent agglutination into masses when subjected to pressure. It should not, however, be too dry to prevent the easy separation of any agglutinated particles under the pressure of the thumb or of a rubber pestle. The sieve should have circular holes punched in a sheet of metal of convenient thickness to give it the requisite degree of strength. Sieves made of wire gauze are not so desirable but it is difficult to get the finer meshes as circular perforations. Such sieves cannot give a uniform product on account of the greater diagonal diameter of the meshes and the ease with which the separating wires can be displaced. It is convenient to have the sieves arranged _en batterie_; say in sets of three. Such a set should have the holes in the three sieves of the following dimensions; _viz._, 1st sieve 2 millimeters diameter. 2nd „ 1 millimeter „ 3rd „ 0.5 „ „ Coarser single sieves may be used to separate the fragments above two millimeters diameter if such a further classification be desired. Each sieve fits into the next finer one and the separation of a sample into three classes of particles may be effected by a single operation. In most cases, however, it is better to conduct each operation separately in order to promote the passage of agglutinated particles by gentle pressure with the thumb or with a rubber pestle. In no case should a hard pestle be used and the pressure should never be violent enough to disintegrate mineral particles. There is much difference of opinion concerning the smallest size of particles which should be obtained by the sieve. Most analytical processes prescribe particles passing a sieve of one millimeter mesh (¹⁄₂₅ inch). There is little doubt, however, of the fact that a finer particle would be better fitted for subsequent analysis by the hydraulic method. For this purpose a sieve of 0.5 millimeter circular mesh is preferred.