=213. General Principles.=—The laws, already discussed, applying to the subsidence of a solid particle in a liquid, are equally applicable to the separation of the particle by imparting a motion to the liquid at a given rate. If a solid particle subside in a given liquid at the rate of one millimeter per second it follows that this particle will remain at rest if the liquid be set in motion upward with a like velocity. If the velocity be greater the particle will be carried upward and eventually out of the containing vessel. Such a particle is said to have a hydraulic value of one millimeter per second. If there be a perfect separation of a soil into its constituent particles and no subsequent flocculation, all the particles of one millimeter hydraulic value and less will be separated by a current of the velocity mentioned. The general principles on which the separation rests, therefore, are the securing of the proper granulation of the sample and the maintenance of a fixed velocity of the current until the separation is finished. The separation must be commenced with a period of subsidence so as to remove first of all the suspended clay or impalpable particles. The velocity can then be increased in a certain fixed ratio to secure a separation into particles of any required hydraulic value. =214. Nöbel’s Apparatus.=—One of the earliest methods of separating the soil particles by a moving liquid is that of Nöbel.[147] The apparatus is shown in Fig. 28. The four separating vessels 1, 2, 3, 4 are of glass, pear shaped, and have a relative capacity of 1³, 2³, 3³, 4³, or 1 : 8 : 27 : 64. No. 4 has an outlet tube leading to the beaker B, of such a capacity as to allow the passage of just nine liters of water in forty minutes, constant pressure being maintained by means of a Mariotte’s bottle or of the constant level apparatus A, _a_, _b_, which is connected with the main water supply through the tube _a_ by means of a rubber hose. The reservoir C should hold about ten liters. The sample of soil to be separated should be previously boiled and passed through a sieve having circular openings one millimeter in diameter. The flask in which the sample is boiled is allowed to stand for some time when the muddy supernatant liquid is poured into elutriator No. 2 and the remaining sediment washed into No. 1. No. 1 is filled with water by connecting it with the water supply and opening the pinch-cock _p_. The water is carefully admitted until the air is all driven out and Nos. 1 and 2 connected. The cock _p_ is then opened and the vessels all filled, and the water allowed to run into B for forty minutes, the level being maintained uniformly at A. [Illustration: FIGURE 28. NÖBEL’S ELUTRIATOR. ]