Sulphate of soda is very soluble in water, and at a temperature of thirty-three (Centigr.) water can dissolve it in the greatest degree. If we pour a layer of oil on a solution saturated with Glauber’s salts, and let it stand, it will not produce crystals; but if we thrust a glass rod through the oil into contact with the solution, crystallization will be instantaneous. This singular phenomenon becomes even more striking when we put the warm concentrated solution into a slender glass tube, A B, which we close after having driven out the air by the bubbling of the liquid (fig. 306). When the tube has been closed, the crystals of sulphate of soda will not form, even with the temperature at zero; nevertheless the salts, being less soluble cold than hot, are found in the fluid in a proportion ten times larger than they would contain under ordinary conditions. If the end of the tube be broken the salt will crystallize immediately. We will describe another experiment, but little known and very remarkable, which exhibits in a striking manner the process of instantaneous crystallizations. Let one hundred and fifty parts of hyposulphite of soda be dissolved in fifteen parts of water, and the solution slowly poured into a test-glass, previously warmed by means of boiling water, until the vessel is about half-full. One hundred parts of acetate of soda is then dissolved in fifteen parts of water, and poured slowly into the first solution, so that they form two layers perfectly distinct from each other. The two solutions are then covered with a little boiling water, which, however is not represented in our illustration. After it has been left to stand and cool slowly, we have two solutions of hyposulphite of soda and acetate of soda superposed on each other. A thread, at the end of which is fixed a small crystal of hyposulphite of soda, is then lowered into the test-glass; the crystal passes through the solution of acetate without disturbing it, but it has scarcely reached the lower solution of hyposulphite than the salt crystallizes instantaneously. (_See_ the test-glass on the left of fig. 307.) We then lower into the upper solution a crystal of acetate of soda, suspended from another thread. This salt then crystallizes also. (_See_ experiment glass on the right of fig. 307.) This very successful experiment is one of the most remarkable belonging to the subject of instantaneous crystals. The successive appearance of crystals of hyposulphite of soda, which take the form of large, rhomboidal prisms, terminating at the two extremities with an oblique surface, and the crystals of acetate of soda, which have the appearance of rhomboidal, oblique prisms, cannot fail to strike the attention and excite the interest of those who are not initiated into these kinds of experiments. [Illustration: Fig. 307.—Experiment of instantaneous crystallization.]