DUST AND DISEASE. 319 



rounded by hydrogen, but the water was not free from 

 floating matter. It is so hard to be clean in the midst of 

 dirt. Here, however, is an approach to pure water. It is 

 from the Lake of Geneva, and the bottle was carefully 

 filled for me by my distinguished friend Soret. The track 

 of the beam through it is of a delicate sky blue ; there is 

 scarcely a trace of grosser matter. 



The purest water that I have seen probably the purest 

 which has been seen hitherto has been obtained from the 

 fusion of selected specimens of ice. But extraordinary 

 precautions are required to obtain this degree of purity. 

 An apparatus was devised and constructed by my assistant 

 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 glass bulb. In the funnel is placed a block of the 

 most transparent ice, and over the funnel is a glass receiver. 

 This is first exhausted and refilled several times with air, 

 which has been 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 face, and 

 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. These two bulbs 

 contain water obtained in this way, the purity of which is 

 the maximum hitherto attained. Still I should hesitate to 

 call the water absolutely pure. When the concentrated 

 beam is sent through it the track of the beam is not invis- 

 ible, but of the most exquisitely delicate blue. This blue 

 is purer than that of the sky, so that the matter which pro- 

 duces 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 sus- 



