Clerk Maxwell suggested yet another experiment in illustration of this law. By means of an ice lens he collected the sunlight to a focus in the middle of a basin of clear water, and observed that no effect was discernible in the water. He then directed the focus (the spot of light) on to a mote in the water. The mote became hot, the water was agitated, convection currents were formed, and the mote was carried up in them. This showed that rays of light from the sun do not affect the substances through which they can pass, and that they heat bodies through which they do not pass. It has been demonstrated by laboratory experiments that all hot bodies emit rays of heat, whether we see the rays or not. When we see the rays the bodies are said to be red or white-hot. The process by which heat passes from one body to another without warming the intervening medium is called radiation. Radiation takes place only through transparent bodies. Rays of heat, like rays of light, pass through transparent bodies; whereas they are absorbed by, that is they make hot, opaque bodies. Heat rays travel in straight lines and are reflected from polished surfaces; their intensity varies inversely as the square of the distance of the object on which they fall from their source. The heat of an ordinary fire is radiant heat; when we sit round the fire we act as opaque bodies and absorb the heat, and are what we call scorched if the fire is very bright. If we move away from the fire, still letting the same firelight shine on us, we are not scorched; this is because the heating power of the rays varies inversely as the distance from their source, therefore if we move away double the distance we receive one quarter of the heat that we received before we moved. If we draw our chairs to one side we are not scorched, because the rays of heat do not travel round a corner. CONDUCTION OF HEAT