SCIENTIFIC USE OF THE IMAGINATION. 123 



the particles of our sky may be inferred from the con- 

 tinuity of its light. It is not in broken patches, nor 

 at scattered points, that the heavenly azure is revealed. 

 To the observer on the summit of Mont Blanc, the 

 blue is as uniform and coherent as if it formed the sur- 

 face of the most close-grained solid. A marble dome 

 would not exhibit a stricter continuity. And Mr. 

 Glaisher will inform you, that if our hypothetical shell 

 were lifted to twice the height of Mont Blanc above 

 the earth's surface, we should still have the azure over- 

 head. By day this light quenches the stars; even by 

 moonlight it is able to exclude from vision all stars be- 

 tween the fifth and the eleventh magnitude. It may 

 be likened to a noise, and the feebler stellar radiance 

 to a whisper drowned by the noise. 



What is the nature of the particles which shed this 

 light? The celebrated De la Rive ascribes the haze of 

 the Alps in fine weather to floating organic germs. 

 Now the possible existence of germs in such profusion 

 has been held up as an absurdity. It has been affirmed 

 that they would darken the air, and on the assumed 

 impossibility of their existence in the requisite num- 

 bers, without invasion of the solar light, an apparently 

 powerful argument has been based by believers in spon- 

 taneous generation. Similar arguments have been 

 used by the opponents of the germ theory of epidemic 

 disease, who have triumphantly challenged an appeal 

 to the microscope and the chemist's balance to decide 

 the question. Such arguments, however, are founded 

 on a defective acquaintance with the powers and pro- 

 perties of matter. Without committing myself in the 

 least to De la Rive's notion, to the doctrine of spon- 

 taneous generation, or to the germ theory of disease, 

 I would simply draw attention to the demonstrable fact, 

 that, in the atmosphere, we have particles which defy 



