80 THE FLOATING-MATTEE OF THE AIR. 



In the funnel was placed a block of ice, I, selected for 

 its transparency, having a volume of 1000 cubic inches 

 or thereabouts, and over the ice was placed an air-tight 

 receiver. Several times in succession the air was re- 

 moved from this receiver, its place on each occasion 

 being taken by other air carefully filtered through 

 cotton-wool. The transparent ice was thus surrounded 

 by moteless air. 



The ice was now permitted to melt; its water trickled 

 into the small glass bulb below, which was filled and 

 emptied a great number of times. From the very heart 

 of the block of ice the water was finally taken and sub- 

 jected to the scrutiny of the concentrated beam. It 

 proved to be the purest liquid I had ever seen — pro- 

 bably the purest human eye had ever seen ; but still it 

 contained myriads of ultra-microscopic particles. The 

 track of the beam through it was of the most delicate 

 blue, the blue light being perfectly polarized. It could 

 be wholly quenched by a Nicol's prism, the beam then 

 passing through the liquid as through a vacuum. A 

 comparison of the light with that scattered by mastic 

 particles such as those above referred to, proved the 

 suspended particles of the ice-water to be far smaller 

 than those of the mastic. No microscope, therefore, 

 could come near them.* Such water, however, was 

 proved by Dr. Sanderson to be as infectious as the water 

 from any ordinary tap. 



Infinitesimal as these particles are, however, they 

 may be separated by mechanical means from the liquid 

 in which they are held in suspension. Filters of porous 

 earthenware, such as the porous cells of Bunsen's battery, 



^ I have endeavoured to convey some notion of the smallness of 

 these scattering particles in * Fragments of Science,' Art. 'Scientific 

 Use of the Imagination.' See note on Mr. Dallinger's observations 

 at the end of this Memoir. 



