150 SCIENTIFIC AMUSEMENTS. 



of lime, which being insoluble in plain water precipitates, and is 

 removed by standing and running off the clear fluid from the top. 

 Any suspended clayey or other solid matter is thus removed, being 

 carried down by the subsiding carbonate of lime ; accordingly the 

 water thus purified is usually of a beautiful blue hue when viewed 

 in large tanks. Sometimes mechanical filtering or straining 

 arrangements are employed instead of simple standing at rest so 

 as to save time. 



Permanently hard water is not materially improved by such 

 treatment, except in so far as bicarbonate of lime (temporary hard- 

 ness) is thereby removed. If, however, carbonate of soda is added 

 to the water, the sulphate of lime present is acted upon by the 

 carbonate of soda, and carbonate of lime formed, which being 

 insoluble precipitates. "Washing soda," "soda crystals," "Scotch 

 soda," " soda ash," &c., are different names for more or less pure 

 carbonate of soda used by laundresses and others for softening 

 hard water and aiding the action of soap. 



Prepare some artificial chalk solution (solution of bicarbonate 

 of lime) as in the last experiment (155), and add to it some clear 

 lime-water; a precipitate will be formed, and if the right 

 quantities of each fluid, are used the whole of the lime will be 

 removed from solution; this, however, is not easy to effect 

 properly without making analyses of each solution, so as to 

 calculate in what proportions they must be mixed. In any 

 case, the formation of a precipitate illustrates Clark's purifying 

 process. 



Expt. 157. Softening permanently Hard Water. Get some 

 plaster of Paris and put it in a bottle nearly filled with distilled 

 water ; shake up at intervals, and then allow the liquid to subside 

 till next day ; pour off some of the clear fluid, and you will have 

 an artificially-prepared excessively hard water, such as is inten- 

 tionally produced for brewing certain kinds of beer, the process 

 of hardening in this way being termed " Burtonising " the 

 water. Add to the hardened water (solution of sulphate of 

 lime) some clear solution of washing-soda crystals and you will 

 at once see that a precipitate forms, showing that some of the 

 lime has been thrown out of solution. One part of the use of 

 soda in washing consists in its power of thus precipitating lime ; 

 besides which, it acts like soap as a "detergent" or cleansing 

 agent. 



Expt. 158. To produce Coloured Precipitates by means of a 

 Gas. Set up a sulphuretted hydrogen generator (Expt. 145), and 

 into different test-glasses pour a little of each of the following 

 solutions : (1) sulphate of copper, (2) acetate of lead, (3) tartar 



