138 FRAGMENTS OF SCIENCE. 



substances might be mentioned whose vapours, when 

 mixed with air and subjected to the action of a solar or 

 an electric beam, are thus decomposed, the products of 

 decomposition hanging as liquid or solid particles in the 

 beam which generates them. And here I must appeal 

 to the inner vision already spoken of. Eemembering the 

 different sizes of the waves of light, it is not difficult to 

 see that our minute particles are larger with respect to 

 some waves than to others. In the case of water, for 

 example, a pebble will intercept and reflect a larger 

 fractional part of a ripple than of a larger wave. We 

 have now to imagine light-undulations of different di- 

 mensions, but all exceedingly minute, passing through 

 air laden with extremely small particles. It is plain 

 that such particles, though scattering portions of all 

 the waves, will exert their most conspicuous action upon 

 the smallest ones; and that the colour-sensation answer- 

 ing to the smallest waves in other words, the colour 

 blue will be predominant in the scattered light. This 

 harmonises perfectly with what we observe in the firma- 

 ment. The sky is blue, but the blue is not pure. On 

 looking at the sky through a spectroscope, we observe all 

 the colours of the spectrum; blue is merely the pre- 

 dominant colour. By means of our artificial skies we 

 can take, as it were, the firmament in our hands and ex- 

 amine it at our leisure. Like the natural sky, the arti- 

 ficial one shows all the colours of the spectrum, but blue 

 in excess. Mixing very small quantities of vapour with 

 air, and bringing the decomposing luminous beam into 

 action, we produce particles too small to shed any sensi- 

 ble light, but which may, and doubtless do, exert an 

 action on the ultra-violet waves of the spectrum. We 

 can watch these particles, or rather the space they occu- 

 py, till they grow to a size able to yield the firma- 

 mental azure. As the particles grow larger under the 



