STRUCTURE AND LIGHT OF THE SKY. 253 



as those which our laboratory experiments reveal. It is 

 not, however, with the iridescences, however beautiful they 

 may be, that we have now to occupy our thoughts, but 

 with other effects which bear upon the two great standing 

 enigmas of meteorology — the color of the sky and the polar- 

 ization of its light. 



It is possible, as stated, by duly regulating the quantity 

 of vapor, to make our precipitated particles grow from an 

 infinitesimal and altogether ultra-microscopic size to masses 

 of sensible magnitude ; and by means of these particles, in 

 a certain stage of their growth, we can produce a blue 

 which shall rival, if it does not transcend, that of the deepest 

 and purest Italian sky. Let this point be in the first place 

 established. Associated with our experimental tube is a 

 barometer, the mercurial column of which now indicates 

 that the tube is exhausted. Into the tube is introduced a 

 quantity of the mixed air and nitrite-of-butyl vapor sufficient 

 to depress the mercurial column one-twentieth of an inch ; 

 that is to say, the air and vapor together exert a pressure 

 of one six-hundredth of an atmosphere. I now add a quan- 

 tity of air and hydrochloric acid sufficient to depress the 

 mercury half an inch farther, and into this compound and 

 highly-attenuated atmosphere I discharge the beam of the 

 electric light. The effect is slow ; but gradually within the 

 tube arises this splendid azure, which strengthens for a time, 

 reaches a maximum of depth and purity, and then, as the 

 particles grow larger, passes into whitish blue. This ex- 

 periment is representative, and it illustrates a general 

 principle. Various other colorless substances of the most 

 diverse properties, optical and chemical, might be employed 

 for this experiment. The incipient cloud in every case 

 would exhibit this superb blue ; thus proving to demonstra- 

 tion that particles of infinitesimal size, without any color of 

 their own, and irrespective of those optical properties ex- 

 hibited by the substance in a massive state, are competent 

 to produce the color of the sky. 



