370 DUST IN THE ATMOSPHERE. 



Specimens of this dust have, on different occasions, 

 been submitted to microscopic examination, and have 

 been found to be composed of infinite numbers of 

 minute infusoria, and particles of decomposed calcarious 

 and feruginous rocks and shells. 



An enquiry into the nature and composition of "dust" 

 has of late years opened up an enquiry into many 

 exceedingly important and curious questions: some of 

 which have been brought prominently into notice by 

 the modern germ theory of disease, and although 

 such considerations are, of course, foreign to our present 

 subject, we may just spare a moment to take a brief 

 glance at the question of these countless myriads of 

 minute particles, which are continually floating in the 

 air around us, sometimes in a visible form, which we 

 call "dust," but more often in an invisible form, so 

 that their existence is unknown and unsuspected by 

 the great majority of mankind. Most of us, however, 

 will at one time or another have seen the still air of 

 a dark room irradiated by a strong ray of light; and 

 the infinite numbers of particles which are floating 

 about in it, all of them in perpetual motion, can hardly 

 have escaped our notice. If we observe them atten- 

 tively, w r e shall see that though some of them appear 

 to be merely floating in the air, there are others which 

 may be seen jumping, rotating, and passing to and 

 fro, in rapid motion; often against the set of the 

 aerial current, or draught, and the motion of these 

 latter particles is of so peculiar a nature, that they 

 can leave no doubt upon the mind of the reflective 

 student that they consist of living organisms yet on 

 admitting the light of day, the atmosphere of that 

 room will appear perfectly pure, and free from dust: 



