The Romance of Modern Chemistry

FACTS ABOUT SOLUTIONS purely academic interest, but they really have a direct bearing on many practical problems. To take one instance: a knowledge of the properties of solutions is essential to any one who attempts to understand either plant or animal life, for the vital processes are invariably associated with solutions. The ultimate unit in the plant is the cell, and the cell sap is the seat of its life; fresh food, too, is brought from outside always in dis- solved form. In the animal, again, solutions are every- where in evidence—to wit, the blood, the digestive fluids, the urine, the lymph. From the biological point of view, in fact, the study of solutions is to be regarded as of the utmost importance. One feature about solutions which is very characteristic, and at the same time fairly easily detected, is the property of diffusion. It must not be supposed that when we dissolve cane sugar in water and set the solution on one side, the sugar molecules remain absolutely at rest. On the contrary, we have every ground for believing that each sugar molecule, surrounded, it may be, by a retinue of water molecules, is constantly moving about through the solution, ever and anon coming into collision with other molecules. We must picture a sugar solution, therefore, as a scene of bustling activity, and the molecules as on the move in every direction, limited only by the boundaries of the liquid ; for they cannot travel except where there is a water-way. In virtue of this molecular movement, it follows that when a strong sugar solution is put in contact with pure water the sugar molecules will gradually distribute them- selves throughout the water. In fact, the process of distribution—“ diffusion,” as it is called—continues until the strength of the sugar solution is everywhere the same. 80S