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soak the weighed piece for one or more days in cold water. The boiling test gives the whole absorption in a very short time, and also eliminates errors caused by surface tension. Langenbeck states that in testing absorption a corner of the piece should be left unimmersed so as not to seal the pores but to leave a means for the air to escape. It is doubtful whether this makes any difference. Crushing strength. A piece of certain length is sup- ported at the two ends, and weights are applied to the middle till the piece ruptures. 80 CERAMIC CHEMISTRY. Freezing test. This refers to the adhesion of the glaze under the strain of water in the body expanding to ice just under the glaze. It is not generally practicable in laboratories to have a freezing chamber, and crystallisation is therefore resorted to. The ware is dipped, glaze first, to the extent of an inch in a saturated solution of some highly crystallising substance, such as aluminium sulphate, ammonium nitrate, copper nitrate, or sodium dichromate. Corrosive tests. The resistance of glaze to a liquid is best studied by applying the liquid to the surface with a brush or, in the case of vessels, by boiling the liquid in them. Resistance to vapour is tested by placing the ware in a bell- jar filled with the vapour. A crucible filled with strong hydrochloric acid placed in a bell- jar will soon detect instability in a glaze. If the glaze is untouched after half