SCIENTIFIC USE OF THE IMAGINATION. 147 



and from a whitish blue it passes to a more or less perfect 

 white. If the action be continued long enough, we end 

 by filling the tube with a dense cloud of sulphur-particles, 

 which by the application of proper means may be rendered 

 visible. 



Here, then, our ether-waves untie the bond of chemical 

 affinity, and liberate a body sulphur which at ordinary 

 temperatures is a solid, and which therefore soon becomes 

 an object of the senses. We have first of all the free 

 atoms of sulphur, which are both invisible and incompetent 

 to stir the retina sensibly with scattered light. But these 

 atoms gradually coalesce and form particles, which grow 

 larger by continual accretion, until after a minute or two 

 they appear as sky-matter. In this condition they are in- 

 visible themselves, but competent to send an amount of 

 wave-motion to the retina sufficient to produce the fir- 

 mamental blue. The particles continue, or may be caused 

 to continue, in this condition for a considerable time, 

 during which no microscope can cope with them. But 

 they continually grow larger, and pass by insensible grada- 

 tions into the state of cloud, when they can no longer elude 

 the armed eye. Thus without solution of continuity we 

 start with matter in the molecule, and end with matter in 

 the mass, sky-matter being the middle term of the series of 

 transformations. 



Instead of sulphurous acid, we might choose from a 

 dozen other substances, and produce the same effect with 

 any of them. In the case of some probably in the case 

 of all it is possible to preserve matter in the skyey con- 

 dition for fifteen or twenty minutes under the continual 

 operation of the light. During these fifteen or twenty 

 minutes the particles are constantly growing larger, with- 

 out ever exceeding the size requisite to the production of 

 the celestial blue. Now, when two vessels are placed be- 

 fore you, each containing sky-matter, it is possible to state 



