ABOUT COMMON WATEK. 335 



In the rain-water basin you have nothing left behind ; 

 in the greensand-water basin you have a small residue 

 of solid mineral matter ; in the chalk-water basin you 

 have a comparatively large residue. The reason of this 

 is that chalk is soluble in rain-water, and dissolves in 

 it, like sugar or salt, though to a far less extent ; while 

 the water of my well, coming from the greensand, 

 which is hardly soluble at all, is almost as soft as rain- 

 water. 



The simple boiling of water is sufficient to precipi- 

 tate a considerable portion of the mineral matter dis- 

 solved in it. One familiar consequence of this is, that 

 kettles and boilers in which haid water is used become 

 rapidly incrusted within, while no such incrustation is 

 formed by soft water. Hot-water pipes are sometimes 

 choked by such incrustation ; and the boilers of 

 steamers have been known to be so thickly coated as to 

 prevent the access of heat to the water within them. 

 Not only was their coal thus wasted, but it has been 

 found necessary in some cases to burn the very spars in 

 order to bring the steamers into port. 



There is no test of the presence of suspended matter 

 in water or air so searching and powerful as a beam of 

 light. An old English writer touched this point when 

 he said : ' The sun discovers atomes, though they be 

 invisible by candle-light, and makes them dance in 

 his beams.' In the purest water it may be filtered 

 water; it may be artificially-distilled water; it maybe 

 water obtained by the melting of the purest ice a 

 sufficiently strong searching beam reveals suspended 

 matter. I have done my best to get rid of it, but can 

 hardly say that I have completely succeeded. 



Differences in quantity are, however, very strikingly 

 revealed. When, in a darkened study, I send a con- 

 centrated beam through our well-water, after boiling, 



