76 THE FLOATING-MATTER OF THE AIR. 



is seen to consist : — first, of particles so coarse that their 

 individual motions can be followed by the eye ; secondly, 

 of a finer matter which is not to be distinguished as 

 motes, but which emits a uniform and changeless light. 

 In this finer matter the coarser motes move as in a 

 medium. 



As regards the production of colour, the action of 

 small particles has been examined by Briicke in a paper 

 ^ On the Colours of Turbid Media.' ^ In relation to the 

 question of polarization, Professor Stokes has made some 

 remarks in his memoir ' On the Change of the Kefrangi- 

 bility of Light.' ^ I miay also be permitted to refer to 

 my own papers ' On New Chemical Reactions by Light ' 

 and ' On the Blue Colour of the Sky,' in the Proceed- 

 ings of the Eoyal Society for 1868-69, and to a paper 

 'On the Action of Eays of High Refrangibility on 

 Gaseous Matter,' in the Philosophical Transactions for 

 1870. M. Soret, Lord Rayleigh, and Mr. Bosanquet 

 have also worked at this subject, which, as far as it now 

 concerns us, a few words will render clear. 



When the track of a parallel beam in dusty air is 

 looked at horizontally through a Nicol's prism, in a 

 direction perpendicular to the beam, the longer diago- 

 nal of the prism being vertical, a portion of the light 

 from the finer matter, being polarized, is extinguished. 

 The coarser motes, on the other hand, which do not 

 polarize the light, flash out with greater force, because 

 of the increased darkness of the space around them. 



The individual particles of the finest floating matter 

 of the air lie probably far beyond the reach of the 

 microscope. At all events it is experimentally demon- 

 strable that there are particles whicli act similarly upon 

 light, and which are entirely ultra-microscopic. A few 



^ Pogg. Ann. Ixxxviii. p. 363. 



2 Philosophical Transactions, vol. 142, pp. 529-530. 



