The radiometer is a heat engine of remarkable delicacy as well as great simplicity. It illustrates a class of phenomena discovered by Crookes which are difficult to explain in a brief and popular way. The instrument consists of a very slight spider of aluminium supporting on the end of each of its four arms a very thin mica plate blackened on one side and silvered on the other. The aluminium spider is provided with a jewel, which rests upon a delicate needle point supported at the center of the glass globe. The spider is retained on its pivot by a small tube extending downward from the top of the globe. When placed in sunlight or near a gas or lamp flame, the vanes revolve rapidly. An alum cell interposed between the radiometer and the source of light and heat allows the light to pass, but intercepts the heat rays. Under these conditions the vane will not rotate. An iodine cell, which is opaque to light, when arranged in the same way allows the heat rays to go through, and these cause the rotation of the vane.