Figure 2 is a cubical box, with a glass front. If a glutinous substance be spread over the bottom and the box allowed to remain quiescent for from five to seven days the dust motes will slowly settle down and attach themselves to the bottom. The air then will be what is called “optically pure.” Now, if it be taken into a dark room and an inclosed lamp at _a_ be allowed to send a beam of light into the window at _b_ and out at _c_, it will be found that the interior remains dark no matter how powerful the light from the lamp. The light is seen to enter and to leave but where it encounters the dust-free air there is nothing to scatter the light rays and they remain invisible to the eye. =Dust Motes Prolong Twilight.= The bending or refraction of light as the sun’s rays pass obliquely through the air at sunrise and at sunset displaces the apparent position of the sun, elevating it by an amount about equal to its own apparent diameter, so that one may see it and receive its light when geometrically it is entirely below the horizon. A little later in the evening and its rays fall upon the upper air too obliquely to be bent down to the earth by refraction; but darkness does not yet ensue, for the rays are scattered by the dust motes and possibly by the molecules of the gases and sent downward from particle to particle, resulting in a soft shimmering light that almost imperceptibly fades away, and which in higher latitudes may last for hours. CHAPTER V LIGHT, HEAT, AND TEMPERATURE MORE WONDERFUL THAN ANY FICTION ARE THE FACT OF INVISIBLE LIGHT, AND THE DIFFERENCE BETWEEN HEAT AND TEMPERATURE The heat that escapes from the earth’s interior is minute in comparison to that received from the sun, which is the main source of the earth’s supply. Heat is manifested by the motions of the molecules of matter, whether solid, liquid, or gaseous. It is transmitted through space in some mysterious manner, for space is practically void of an atmosphere. One cannot conceive of motion taking place in a void, for there is nothing to move. Therefore it is assumed that interstellar space must be filled with a transmitting medium; to this the name of ether has been given. Nothing is known of its structure, but it is believed that it penetrates all bodies and fills the space between their molecules.