A very simple experiment will serve to reconcile the apparent contradiction of the universal law by the observed fact. Suppose we have two fluids, oil and water, of which oil is, bulk for bulk, lighter than water. If the oil be poured into a glass beaker, it will be seen to rest at the bottom of the beaker; if water be now poured into the same beaker the water will go to the bottom of the beaker and will displace the oil and lift it up so that the oil will float on the water; the oil may be lifted to any height we please if sufficient water be poured in to lift it to that height. If a single drop of oil be introduced into the water by means of a pipette and be liberated at the bottom of the beaker the water will close in under it, and lift it up to the surface. In both cases the oil "rises" through the water. Oil, however, has no tendency to "rise" by itself, and in this case it lay motionless until it was lifted by the heavier fluid. We may use colloquial language when describing phenomena if we bear in mind what is really taking place. A balloon "rising" through the air is exactly analogous to the drop of oil in the water. The balloon is, bulk for bulk, lighter than air; the air therefore closes in under it and lifts it just as the water lifted the bubble of oil. EFFECTS OF CHANGES OF TEMPERATURE ON AIR Let us apply this to air. Air when warmed expands, and therefore warm air is, bulk for bulk, lighter than cold air. Warm air behaves in the presence of cold air as the balloon: it is displaced and lifted by the cold air, the result being an ascending stream of warm air, which is called a convection current. The movement of ascending smoke is essentially the same as that of the warmed air. Smoke is warm air made visible by the particles of soot with which it is laden. The particles of soot would fall to the ground except that they are carried upwards in the stream of warm air. Dr. W. N. Shaw has called attention to the importance of these phenomena in his book on "Air Currents and the Laws of Ventilation," in the Cambridge Series of Physical Text-books. He there says: "The dominant physical law in the ventilated space is the law of convection. It is at once the condition of success and the cause of most failures. Without convection, ventilation would be impossible; in consequence of convection, nearly all schemes of ventilation fail. "The law of convection is the law according to which warmed air rises and cooled air sinks in the surrounding air. Its applications are truly ubiquitous. Every surface, _e.g._ a warm wall, or a person warmer than the air in the immediate neighbourhood, causes an upward current; every surface colder than the air in contact with it causes a downward current. "Ventilation would be much easier if warmed air or cooled air could be carried along at any height required; but the law of convection is inexorable: warmed air naturally finds the ceiling, cooled air the floor."