_Extraction of solid materials_, such as barks, or valonia, is more difficult, but the following is a convenient method, which has been recognised as official by the International Association. An ordinary beaker, of about 200 c.c. capacity, but which may be varied in size according to the weight of the material which it is necessary to treat, is placed in a water-bath, as shown in Fig. 65. A thistle-headed funnel, the stem of which is bent twice at right angles, and of which the head is covered with a piece of fine silk gauze (such as is used by millers) to act as a strainer, is placed in the beaker and held in position by a clamp as shown in the figure. To the free end of its stem a piece of glass tube, six or eight inches long, is attached by indiarubber tube, which is provided with a pinchcock to regulate the flow of liquid. Fine silver-sand, freed from iron and soluble matters by washing first with hydrochloric acid, and then very thoroughly with water, is now poured into the beaker, so as to surround the head of the funnel to about half an inch in depth; and the weighed quantity of tanning material is next introduced. It is best to cover the material with cold water, and allow it to stand all night, but in case of haste, water of 30° to 50° may be used, and the extraction proceeded with after the material is thoroughly soaked. Percolation is started by sucking the syphon, and allowing the liquid to drop slowly into a liter flask, the temperature of the water-bath being maintained by a Bunsen burner, and the beaker being refilled as it requires it with water at the desired temperature.[155] At least 500 c.c. must be percolated before the temperature is allowed to exceed 50°, after which, except in the case of sumach and canaigre, which should be begun about 30°, and at no time allowed to rise above 50°, the temperature may be raised to boiling point. At least 1¹⁄₂ hour should be employed in percolating 800 to 900 c.c. and if the material is not then practically exhausted, the liter flask must be withdrawn, and an ordinary ungauged flask substituted, into which the percolation is continued till the material is exhausted. The very dilute liquor in the second flask is now boiled down till its volume is sufficiently small to be added to that in the liter flask, a small funnel being placed in its neck during ebullition, to prevent spirting and the access of air. Under no conditions must the stronger liquor of the first part of the percolation be boiled down, as this would involve destruction of tannin. The solution is now cooled, and made up to the mark as has been before described. Most ordinary materials may be practically exhausted by the liter of water if percolation is slow, and the trouble of evaporation may thus be avoided. [155] The material should be kept in an even layer, and if necessary the surface may be stirred at intervals with the thermometer or a glass rod.