SCIENTIFIC USE OF THE IMAGINATION. 121 



cation of proper means may be rendered individually 

 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 there- 

 fore soon becomes an object of the senses. We have 

 first of all the free atoms of sulphur, which are incom- 

 petent 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 individually invisible; but collec- 

 tively they send an amount of wave-motion to the 

 retina, sufficient to produce the firmamental 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 grow 

 slowly larger, and pass by insensible gradations 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 atom, 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 a 

 dozen other substances, and produce the same effect 

 with all of them. In the case of some probably in 

 the case of all it is possible to preserve matter in the 

 firmamental condition for fifteen or twenty minutes 

 under the continual operation of the light. During 

 these fifteen or twenty minutes the particles constantly 

 grow larger, without ever exceeding the size requisite 

 * M. Morren was mistaken in supposing that a modicum of 

 sulphurous acid, in the drying tubes, had any share in the produc- 

 tion of the 'actinic clouds ' described by me. A beautiful case 

 of molecular instability in the presence of light is furnished by 

 peroxide of chlorine as proved by Professor Dcwar. 1878. 



