150 FRAGMENTS OF SCIENCE. 



the summit of Mont Blanc the blue is as uniform and co- 

 herent as if it formed the surface of the most close-grained 

 solid. A marble dome would not exhibit a stricter con- 

 tinuity. And Mr. Glaisher will inform you that if our hy- 

 pothetical shell Avere lifted to twice the height of Mont 

 Blanc above the earth's surface, we should still have the 

 azure overhead. Everywhere through the atmosphere those 

 sky-particles are strewn. They fill the Alpine valleys, 

 spreading like a delicate gauze in front of the slopes of 

 pine. They sometimes so swathe the peaks with light as 

 to abolish their definition. This year I have seen the 

 Weisshorn thus dissolved in opalescent air. By proper 

 instruments the glare thrown from the sky-particles against 

 the retina may be quenched, and then the mountain which 

 it obliterated starts into sudden definition. Its extinction 

 in front of a dark mountain resembles exactly the with- 

 drawal of a veil. It is the light then taking possession of 

 the eye, and not the particles acting as opaque bodies, that 

 interferes with the definition. By day this light quenches 

 the stars ; even by moonlight it is able to exclude from 

 vision all stars between the fifth and the eleventh magni- 

 tude. It may be likened to a noise, and the 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 numbers, without invasion of the 

 solar light, a powerful argument has been based by be- 

 lievers in spontaneous generation. Similar arguments 

 have been used by the opponents of the germ theory of 

 epidemic disease, who have triumphantly challeged an ap- 

 peal to the microscope and the chemist's balance to decide 



