The Romance of Modern Chemistry

FACTS ABOUT SOLUTIONS stronger. A blood corpuscle is affected in exactly the same way when it is put in a strong solution of salt; water actually passes out through the skin, the corpuscle shrinks in size, becomes more dense, and sinks to the bottom of the salt solution. If we were to test a number of sugar solutions of gradually increasing strength in the same way as has been suggested for salt, we should find that the blood corpuscles were burst in all sugar solutions up to 5 per cent, strength, but sank to the bottom in solutions of greater concentration. For both sugar and salt, there- fore, we can, with the help of the corpuscles as indicators, pick out solutions which have the same osmotic effect. From this point of view, a 5 per cent, solution of cane sugar must be regarded as equivalent to a 0’5 per cent, solution of common salt, even although the amounts of dissolved matter are so different in the two cases. This may seem rather strange, but investigations which we cannot consider here have shown that the magnitude of the osmotic pressure is determined, not by the weight of a substance which is dissolved in a given volume of solution, but by the number of molecules present in that volume. Now the sugar molecule is a very heavy one, so that proportionately more of this substance must be taken in order to get a definite number of molecules. There are other interesting properties of solutions which are in reality closely connected with osmotic pres- sure. There is, for instance, the fact, perhaps already known to some readers, that a solution freezes at a lower temperature than water, and boils at a higher temperature. More than that, the extent to which the freezing-point of a solution is below 32° Fahrenheit, and its boiling- point above 212°, is proportional to the number of 307