Osmotic growths may be obtained from a great number of chemical substances. The most easily grown are the soluble salts of calcium in solutions of alkaline phosphates and carbonates, to which we have already alluded. We may also reverse the phenomenon by growing phosphates and carbonates in solutions of calcium salts, but in this case the osmotic growths are not so beautiful. The various silicates play an important part in the constitution of shells and of the skeletons of marine animals. Most of the metallic salts, and more especially the soluble salts of calcium, give rise to the phenomenon of osmotic growth when sown in solutions of the alkaline silicates. In this way, by using different silicates and varying the proportions and the concentrations, we may obtain an immense variety of osmotic growths. A good solution to commence with is the following:-- Silicate of potash, sp. gr. 1.3 (33deg Beaume) 60 gr. Saturated solution of sodium carbonate 60 gr. Saturated solution of dibasic sodium phosphate 30 gr. Distilled water make up to 1 litre. {126} A fragment of fused calcium chloride dropped into this solution will produce a rapid growth of slender osmotic forms which may attain a height of 20 or 30 centimetres. Small pellets may also be made of one part of sugar and two of copper sulphate and sown in the following solution, which must be kept warm until the growth is complete:-- Ten per cent. solution of gelatine 10 to 20 c.c. Saturated solution of potassium ferrocyanide 5 to 10 c.c. Saturated solution of sodium chloride 5 to 10 c.c. Warm water (32deg to 40deg C.) 100 c.c. In this solution we can obtain osmotic growths which may attain to a height of 40 centimetres or more, vegetable forms, roots, arborescent twigs, leaves, and terminal organs. These growths are stable as soon as the gelatine has cooled and set, and may be carried about without fear of injury (Fig. 35). Precipitated osmotic membranes are very widely distributed in nature. Professor Ulenhuth has seen iron growths in alkaline sodium hypochlorite (Javelle water), and Lecha-Marzo has demonstrated the osmotic growth of the various {127} stains used for microscopy, in the liquids used for fixing preparations. [Illustration: FIG. 37.--Osmotic vermiform growth. (_a_) The sickle-shaped growth. (_b_) The growth broken by the upward pressure of the solution. (_c_) The wound having cicatrized, the stem continues to grow downwards. ] We now know that the physical force which builds up these growths is that of osmotic pressure, since the slightest consideration will show the inadequacy of the usual explanation that the growth is due to mere differences of density, or to amorphous precipitation around bubbles of gas. These may indeed affect the phenomenon, but can in no way be regarded as its cause.