ON DUST AND DISEASE. 171 



structed for this purpose : Through the plate of an air- 

 pump passes the shank of a large funnel, attached to 

 which below the plate is a clean glass bulb. In the 

 funnel is placed a block of the most transparent ice, and 

 over the funnel a glass receiver. This is first exhausted 

 and refilled several times with air, filtered by its passage 

 through cotton- wool, the ice being thus surrounded by 

 pure moteless air. But the ice has previously been in 

 contact with mote-filled air ; it is therefore necessary to 

 let it wash its own surface, and also to wash the bulb 

 which is to receive the water of liquefaction. The ice is 

 permitted to melt, the bulb is filled and emptied several 

 times, until finally the large block dwindles to a small 

 one. We may be sure that all impurity has been thus 

 removed from the surface of the ice. The water ob- 

 tained in this way is the purest hitherto obtained. Still 

 I should hesitate to call it absolutely pure. When 

 condensed light is sent through it, the track of the 

 beam is not invisible, but of the most exquisitely delicate 

 blue. This blue is purer than that of the sky, so that 

 the matter which produces it must be finer than that of 

 the sky. It may be> and indeed has been, contended 

 that this blue is scattered by the very molecules of the 

 water, and not by matter suspended in the water. But 

 when we remember that this perfection of blue is 

 approached gradually through stages of less perfect blue ; 

 and when we consider that a blue in all respects similar 

 is demon strably obtainable from particles mechanically 

 suspended, we should hesitate, I think, to conclude that 

 we have arrived here at the last stage of purification. 

 The evidence, I think,' points distinctly to the conclusion 

 that, could we push the process of purification still 

 farther, even this last delicate trace of blue would 

 disappear. 



