SCIENTIFIC USE OF THE IMAGINATION 129 



waves. Enclosing sulphurous acid in a suitable vessel, 

 placing it in a dark room, and sending through it a pow- 

 erful beam of light, we at first see nothing: the vessel 

 containing the gas seems as empty as a vacuum. Soon, 

 however, along the track of the beam a beautiful sky-blue 

 color is observed, which is due to light scattered by the 

 liberated particles of sulphur. For a time the blue grows 

 more intense; it then becomes whitish; and ends in a 

 more or less perfect white. When the action is continued 

 long enough, the tube is filled with a dense cloud of sul- 

 phur particles, which by the application of proper means 

 may be rendered individually visible.* 



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

 ical affinity, and liberate a body — sulphur — which at ordi- 

 nary 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 individ- 

 ually invisible; but collectively they send an amount of 

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

 mental 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 

 Blowly larger, and pass by insensible gradations into the 

 state of cloud, when they can no longer elude the armed 



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

 in the drying tubes, had any share in the production 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 Dewar. 1878. 



