MECHANICAL PRESSURE OF LIGHT; ITS CONSEQUENCES. 237 



larger than the critical size, will fall slowly back towards 

 the sun constituting the prominences; if smaller than the 

 critical value, they will be driven away from the sun forming 

 the curious streams of the corona. If they are, practically, 

 just the size which the light can support they will float 

 permanently suspended and will constitute the main body 

 of the corona. Even the mysterious " hairy structure " of 

 the corona is explicable as due to the supporting power of 

 light on particles of different sizes. 



THE ZODIACAL LIGHT. 



Just after twilight on any clear evening in winter or 

 spring there may be seen on the western horizon a faint 

 soft beam of light. This beam seems to proceed out from 

 each side of the sun to some distance beyond the earth's 

 orbit. It is called the zodiacal light. Its cause has been 

 another of the " mysteries" of astronomy, though it finds 

 an easy present-day explanation in terms of our theory. 

 It was shown, page 51, that incandescent carbon and metals 

 gave off negatively electrified particles or corpuscles a 

 thousand times smaller than the smallest atom. We know 

 that enormous quantities of carbon exist in the photosphere 

 of the sun, and exist, moreover, at a temperature vastly 

 greater than any known on earth. This carbon must emit 

 corpuscles, and, since the corpuscles are almost infinitely 

 small, the effect upon them of the mechanical pressure of 

 light must be extreme. The sun must, therefore, bombard 

 all space with corpuscles travelling with an immense ve- 

 locity. These corpuscles, which it should be remembered 

 are negatively electrified, when they strike the outer regions 

 of the earth's atmosphere will charge it also negatively, and 

 when this charge reaches a certain value the oncoming rush 

 of corpuscles will be deflected by the similar electrification 



