ABOUT COMMON WATER. 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 hard water is used become 

 rapidly incrusted within, while no such incrustation is 

 formed by soft water. Hot-water pipes are sometimes 

 choked by such incrustations ; 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. Xot only 

 was their coal thus wasted, but it has been found neces- 

 sary 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 may be 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, 



