On the other hand, oil and water will not mix, however much you may stir the two together; and the oil, being the lighter, rises to the top as soon as the fluid is quiet. Nor will quicksilver and water mix, but the quicksilver, being very much heavier than the water, rushes to the bottom of the vessel into which the two are put. Neither will sand nor iron filings mix with water; as heavier bodies, they also sink to the bottom. Nor does powdered ice, though it is water in another shape, mix with ice cold water; as a lighter body it floats at the top.
52. =Mixture followed by Increase of Density; Alcohol and Water.=
Strong spirit, or =alcohol=, is a clear transparent fluid which looks like water, but is a very different substance. For example, it boils at a much lower temperature, it burns with a blue flame, it has intoxicating properties, and, like oil, it is very much lighter than water. Hence, if coloured spirit is poured gently upon the surface of water the spirit rests upon the water. Suppose, now, that we take a tall measure graduated into ten equal parts. Fill the lower five with water, and then, very gently, pour in the strongest alcohol, coloured in some way, until the tenth mark is reached. We shall have five volumes of water below, and an equal quantity, or five volumes, of coloured alcohol above. Where the two are in contact, the colour will be diffused into the water for a short distance, but not far, showing that only a slight mixture is taking place. This, however, is not because the two fluids mingle with difficulty; for, with slight stirring, they mix completely, and you have a fluid the colour of which is about half as intense as that of the alcohol, and many of the other properties of which are intermediate between those of pure alcohol and those of pure water.
Thus far, nothing further than simple mixture, as when coloured water was added to pure water, seems to have occurred; but, in reality, something more has happened. In the first place, the mixture is a good deal warmer than either of its components; that is to say, =heat= has been =generated=. In the second place, if you measure the volume of the whole fluid after it has cooled, it no longer stands at the mark =ten=, but distinctly lower, or about =nine and three-quarters=. As the volume of the mixture is less than the sum of the volumes of its two components, it follows that the =density= of the mixture must be =greater= than a density midway between that of the water and that of the alcohol. In other words, the molecules in the mixture do not occupy the same space as they did when they were separate. The result is the same as if the ten volumes had been compressed until they occupied only nine and three-quarters; so that the effect is a contraction similar to that which would be brought about by taking away heat from the mixture. In fact, as we have seen, the mixture gives out a quantity of heat.