The Romance of Modern Chemistry

FACTS ABOUT SOLUTIONS naturally do—that is, diffusing into the water. As the water and the sugar solution are bound to come into equili- brium somehow or other, and as the usual way of reaching equilibrium is barred, the principle of Mahomet and the mountain comes into play. Instead of the sugar molecules diffusing into the water, the latter percolates through the membrane into the sugar solution, and the membrane, if unsupported, would soon be ruptured. The diffusive force of the sugar thus assumes the guise of a water- attracting force. This force is known as the “ osmotic pressure * of the sugar solution, and although it is rather a difficult quantity to measure, several successful attempts have recently been made in this direction. The semi-permeable membrane used in these interesting experiments consisted of copper ferrocyanide, deposited on and supported by the walls of a porous pot or tube. The necessity of giving the membrane some such support will be obvious when it is stated that the osmotic pressure of a 12 per cent, sugar solution is 142 pounds per square inch. A weaker solution has a smaller osmotic pressure; and, in fact, it has been found that this quantity is proportional to the concentration of the solution. Semi-permeable membranes are not only produced by the chemist in his laboratory; they occur frequently in the plant and animal worlds. A red blood corpuscle, for instance, consists of a delicate, flexible, semi-permeable skin, inside which is a solution of the colouring matter of the blood, the hæmoglobin. While the latter is unable to pass out through the enclosing membrane, water can pass in and out freely. The corpuscle is, therefore, exactly comparable with a drop of a sugar solution surrounded by a semi-permeable membrane. 305 u