The Romance of Modern Chemistry

FACTS ABOUT SOLUTIONS Suppose, now, we put some blood corpuscles in pure water; what result may be expected ? Obviously, if the contents of the corpuscle were able to penetrate their skin, they would diffuse out into the surrounding water. Owing, however, to the semi-permeable nature of the enclosing membrane, this is impossible. What really happens is that the water passes in through the skin, which accordingly expands as the contents increase in bulk. The membrane enclosing a blood corpuscle is, however, very delicate; a slight increase in the volume of the contents is sufficient to burst it, so that the hæmoglobin escapes and imparts its colour to the water; the corpuscles are said to be “ laked.” If, now, instead of using pure water we put blood corpuscles in each of several solutions of common salt of gradually increasing strength, we should obtain a very interesting result. In all solutions of less than 0’5 per cent, strength the corpuscles behave as in pure water, bursting and colouring the liquid. In all solutions con- taining more than 0’5 per cent, of salt, the corpuscles sink to the bottom, and leave a colourless liquid above. The explanation of this latter behaviour will be readily understood if we consider what would be the result of putting a drop of sugar solution, surrounded by a semi- permeable membrane, in a still stronger solution. There is a natural tendency, constantly at work, to equalise the osmotic pressures on the two sides of such a mem- brane, so that water will always pass from the solution with the smaller osmotic pressure (that is, from the weaker solution) to the one with the greater osmotic pressure (that is, the stronger solution). In the case suggested, therefore, water will pass from the inside of the drop to the outside, so that the sugar solution within becomes 306