22 THE WORLD'S LUMBER ROOM. 



So much, then, for the dust of the air ; but we have yet 

 to say something about the dust of water ; and here again, 

 though the amount of impurity may be so infinitesimal as 

 to be hardly capable of expression in numbers, and though 

 the individual particles suspended may utterly elude the 

 microscope, yet their presence here, as in the air. is re- 

 vealed to us by light. 



Good drinking-water appears simply dirty in the electric 

 beam, and even that which has been filtered through 

 charcoal is seen to be thick with fine suspended matter ; 

 but by far the greater number of particles are not only 

 invisible to the naked eye, but are beyond the reach of 

 the microscope, and reveal their presence only by colour. 

 In the purest water, obtained, with extra precautions against 

 contamination by the air, from the middle of a block of 

 ice, the electric beam when passed through it appeared of 

 a delicate blue, purer than that of the sky, and therefore 

 produced by particles finer than those suspended in the 

 air. All the evidence, however, points to the conclusion 

 that in perfectly pure water the last trace of colour would 

 disappear, and the beam would be as invisible as it is in 

 perfectly pure air. 



The River Rhine flows into Lake Constance muddy 

 with the sediment it has brought from the mountains ; but 

 on emerging partially filtered at the other end it is of a 

 dark but transparent green. The water of the Lake of 

 Brientz is also a deep transparent green, but that of the 

 Lake of Thun, which it feeds, being more perfectly filtered, 

 is a clear blue. 



Sea-water of a yellow-green Professor Tyndall found 



