STRUCTURE AND LIGHT OF THE SKY. 259 



waves of light. The mixed air and vapor are sufficient to 

 depress the mercurial column one inch. I add to this mix- 

 ture air, which has been permitted to bubble through dilute 

 hydrochloric acid, until the column is depressed thirty inches : 

 in other words, until the tube is full. And now I permit 

 the electric beam to play upon the mixture. For some 

 time nothing is seen. The chemical action is doubtless 

 progressing, and condensation is going on ; but the con- 

 densing molecules have not yet coalesced to particles suffi- 

 ciently large to reflect sensibly the waves of light. As 

 before stated — and the statement rests upon an experimental 

 basis — the particles here generated are at first so small that 

 their diameters would probably have to be expressed in 

 millionths of an inch ; while to form each of these particles 

 whole crowds of molecules are probably aggregated. Helped 

 by such considerations the intellectual vision plunges more 

 profoundly into atomic Nature, and shows us, among other 

 things, how far we are from the realization of Newton's 

 hope that the molecules might one day be seen by micro- 

 scopes. While I am speaking, you observe this delicate blue 

 color, forming and strengthening within the experimental 

 tube. No sky-blue could exceed it in richness and purity ; 

 but the particles which produce this color lie wholly beyond 

 our microscopic range. A uniform color is here developed, 

 which has as little breach of continuity — which yields as 

 little evidence of the particles concerned in its production, 

 as that yielded by a body whose color is due to true mo- 

 lecular absorption. This blue is at first as deep and dark 

 as the sky seen from the highest Alpine peaks, and for the 

 same reason. But it grows gradually brighter, still main- 

 taining its blueness, until at length a whitish tinge mingles 

 with the pure azure ; announcing that the particles are now 

 no longer of that infinitesimal size which mainly scatters 

 the shortest waves. 1 



1 Possibly a photographic impression might be taken long before the 

 blue becomes visible, for the ultra-blue rays are first reflected. 



