SCIENTIFIC USE OF THE IMAGINATION. 121 



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 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 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 micro- 

 scope 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 

 to the production of the celestial blue. Now when two 

 vessels are placed before us, each containing sky-matter, 

 it is possible to state with great distinctness which vessel 

 contains the largest particles. The eye is very sensi- 

 tive to differences of light, when, as in our experiments, 

 it is placed in comparative darkness, and the wave- 

 motion thrown against the retina is small. The larger 

 particles declare themselves by the greater whiteness 

 of their scattered light. Call now to mind the obser- 



