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XV. — On the Formation of Small Clear Spaces in Dusty Air. By 

 Mr John Aitken. (Plate XXXVIII.) 



(Received December 27, 1883 ; read January 21, 1884.) 



The dust particles floating in our atmosphere are every day demanding 

 more and more attention. As our knowledge of these unseen particles in- 

 creases, our interest deepens, and I might almost say gives place to anxiety, 

 when we realise the vast importance these dust particles have on life, whether 

 it be those inorganic ones so small as to be beyond the powers of the 

 microscope, or those larger organic ones which float unseen through our 

 atmosphere, and which, though invisible, are yet the messengers of sickness 

 and of death to many — messengers far more real and certain than poet or 

 painter has ever conceived. 



As the great importance of these dust particles is gradually being realised, 

 we are from time to time increasing our efforts to protect ourselves from these 

 invisible enemies. Professor, now Sir Joseph, Lister has shown us how to 

 contend successfully with those organic germs, which, falling on our wounds, 

 there find a suitable resting-place, and, if not killed, germinate and grow to 

 our destruction. Sanitary societies are every day being formed, one of whose 

 objects is to combat these floating particles by better appliances directed 

 towards the prevention of the conditions suitable for the germination, growth, 

 and increase of these germs, and against their spread from infected centres, 

 while other societies are directing their energies against the artificial produc- 

 tion of those inorganic forms of dust which pollute our atmosphere. 



The immense importance of everything connected with dust must be my 

 excuse for bringing before this Society observations on phenomena which I fear 

 must appear to many as trivial and uninteresting; as the clear spaces to 

 which I shall direct attention are on almost a microscopic scale, and require to 

 be magnified to enable us to see them clearly. 



Professor Tyndall has made many experiments on the light-reflecting par- 

 ticles floating in our atmosphere. He found these particles were destroyed by 

 heat, and that by placing a flame under a brilliant beam of light, which revealed 

 by illuminating the dust in the air, there was seen rising from the flame wreaths 

 of darkness resembling intensely black smoke. lie then found it was not 

 necessary to burn the particles to produce this stream of darkness. This was 

 observed when a hot metal ball was placed under the beam of light, and per- 

 mitted to remain till its temperature had fallen below that of boiling water. It 



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